It looks a lot like an invasion if you are distant, hairy barbarians who are located on the periphery of the Great Mongolian Nation.
Well, no, Lamech. Nukes just hurt people, as opposed to the fires of Hell sending Souls screaming down into the Pit of your Master, which is far worse then any silly "nuclear winter". Thanks to the Gift, people don't like magi as a group, and once you start destroying armies with fire from the sky, no amount of "we meant well" from mind controlled face-men is going to stop the Crusade. Then there's the envy part of the equation. The surrounding kingdoms cannot send fire from the sky, and people being people, will stop at little to get a counter, which is going to lead to a magi arms race or, of course, a Wizards War against the Covenant that broke the rule against dealing with Mundanes and got everyone spun up...... And your Rego Mentum spells will, in canon, be stopped by the Dominion, which protects the leadership of Christendom and the Church, and trumps the Magic Realm. And the system notes are pretty clear that the protect increases depending on the power level of your game. Your game may vary, of course, but that's why it doesn't happen in canon, which is what you asked, yes?
I agree with the OP. Hermetic magic is too powerful for the setting, and one aspect of this is that defeating mundane armies - or, for that matter, angry peasant mobs, fortifications, or so on - isn't that difficult.
One response to that is that the Order prevents magi from doing this sort of thing. If that works for you - great. For me, it creates a game of legalese and annoying Quaesitors that symie the player's every move; I hate that. I also don't think it makes sense, not with Mythic History's all-too-mundane past. Such an arrangement might make sense in a high-fantasy setting where the Empire of Wizards crumbled in devastating internal squabbles and the survivng Order of Wizards was founded so as not to repeat the mistakes of the past; this is a common trope in D&D (going back to the earliest setting, Greyhawk; continuing with Netheril in the Forgotten Realms, and numerous other examples), but without it I find the Order's laws just don't really make sense in-setting and are annoying in-game.
Another response is that the Divine will sort you out. Really ? That holy man with True Faith and - oh, shudder - Magic Resistance 10 ? Even using the extended rules for holy supernatural traditions from RoP:D won't change this. I once statted up the Templars as an order of Mythic Companions using Divine methods and powers; bottom line - even with me fudging the rules in their favor, the Templars still shouldn't be able to defeat a strong covenant, let alone pose a threat to the Tribunal or the Order.
Now direct intervention by God himself trumps everything, yes. But this really just means direct intervention by the SG to desperately save his setting from breaking-up. I hate that even more than I hate the legalese. Barring direct SG intervention, holy men and their angel and saint supporters are just hedge wizards and petty spirits, no match for an Hermetic magus. The angels and saints can at least cower behind the Lunar Sphere to save their ass, and there are some broken mechanics (like archangels being able to duplicate themselves infinitely many ties...), but overall the Divine isn't a threat for Hermetic power. Not realy.
The same goes for the other Realms. Stellatus, the most powerful magical being this side of the Magic Realm, is agame for the season's hunt. Give a magus the True Name of one of hell's lieutenants, those Infernal Might 75 beings, and he'll summon it and Demon Eternal Obliviate is ass. Good thing Satan himself dones'nt have stats, or he'll be toast too.
And I haven't even statred talking about leveling cities, cursing kingdoms, and creating grain with magic...
Hermetic magic is broken. It's too powrful for the setting. You can ignore or legalese that and if that works for you great. If you want the setting to make mre sense power-level wise, with magi fearing mundanes and th Divine and all the supernatural hosts - the you should really greatly lower thep ower level of magi. By a lot.On multile lvels and aspects.
Yair
Edit: Sorry for the spelling errors; I'm writing from Internet Explorer, and it's giving me troubles.
Also - I once statted out the Mongols as led by diabolists. It works fairly well - the Mongols are basically stooges, and the REAL battle is between the clandestine diabolist coven and their numerous allies (forced or duped) and the covenant. The coven can actually do more than Divine magic can, including lots of Magic Resistance on a large scale. I published in Sub Rosa.
When the Mongol Horde invaded real world Hungary the first time, they couldn't take any of the stone castles, or stone fortified towns. It's also why the King of Hungary gave a heap of his lands to his great nobles on the condition they fortified in stone. Even after the Battle of Mohi (1241), where the Hungarians lost the power to field an army in resistance ot the Mongols and left them with free choice of sites to besiege, the Mongols had terrible trouble taking anything of critical value, and within a year after they had left, Hungarian was again successfully able to field armies against its neighbours, because all of its major towns, which were the centre of its economy, were intact.
In Russia, where they held on, their main method was getting Russian loyalists to do the hard work for them.
So, yes, Hermetic magic can probably wreck the Mongols, because frankly they are less tough than imagined, and because their war machine is based on horse archery, which can be opposed by simply destroying the grass for miles around, or fortifying.
The Order can defeat the Mongols without the sort of flashy "rain of fire" magic you are suggesting. All it really needs to do is put an avalanche or two in some of the key Transylvanian passes. Lacking that, all it needs to do is flood the Great Plain for long enough for the horses to die (say, a fortnight).
So, yes, IMO the Battle of Mohi goes a different way in ME, unless the Mongolsa give up on "All magicians and craftsmen are our slaves" thing. As worshippers of a Magical elemental god, facing off against the Order's atheists, there's a possible deal there.
The question in Ars is not usually "Can I do this?". With enough lab time, and vis, you can probably whatever it is. The question is "What does it mean for me to do this? Do I accept the consequences of this? Does it change the way the world works in an interesting way? Am I cuplable for the unintended consequences?" In that sense its a more intimate game than, say, D&D, which really is about killing bigger and bigger things.
The Order did do nukes once: the Battle of the False Sun was basically the analogue, wasn't it?
Wait an order full of mythic companions?!? And you weren't able to make them a threat to the Order? I direct you to the Ceremony ability. Get a few dozen people with decent ceremony and as many people with high (com/int/sta) as you can gather. You should start by granting absurd MR, immunity to damage, all the deprivations, the ability to pass through matter, and immunity to baneful effects under say... level 100. Then make yourself an army of might 200 divine creatures and start blasting random covenants.
Lord Marsune could whip a chunk of ice several miles across at a covenant. (create water Base 3 , +1 rego req +1 Touch +14 increased size)=15th magnitude. Not magical ice mind you, real ice created with the equivalent of a ritual effect. And then he can do it 9 more times. Or he could use the group modifier and launch mile wide chunks of ice ten at a time.
Spirits in the magic realm by in large can't even be targeted by magi. On the other hand, they are free to kill magi and attack by destroying the corresponding vestiges in the magic realm.
Infernists are even nastier than Divine people when ceremony comes out. Also corrupted abilities. I can build a companion grade character that with the help of a say a dozen loyal helpers can score arbitrary levels of ability.
I think its actually better balanced than a lot of the other things. Sure its powerful and can level cities. So can plenty of other things. However, I think magic trounces the mundane. That's okay. What makes me do a double take is when those same mundane forces are presented as anything approaching a threat. These are magi, if they the covenant is taken out by mundane forces they did something wrong.
The Gift doesn't affect people if they aren't in the presence of the magi. As long as the magi are kept out of sight the Gift is not a problem.
Yeah... that's not how I saw it, at least. As I recall, divine Ceremony is rather limited, divine Methods suck in that casting spells becomes very rare, and the guidelines suck in that good effects have prohibitive levels. And if you want to keep reasonable ability scores and reasonable Methods/Powers in the tradition, well... you end up with not-so-impressive supernatural powers. At least I did. Maybe I did something wrong, but I got Magic Resistance 40 (ok - something like 55 with faith points and what not), for example, not 100; Not that MR 100 would be such an insurmountable obstacle for the Order. And none of that "immunity to damage" uber powers.
Yeah, he can exploit the loophole in the magic resistance rules to throw (mundane) ice at you. And he can do some fairly powerful spells; up to level 75 - in ice only.
In my opinion - as a Might 50 creature Lord Marsyne should be defeatable by Hermetic magic, but only just barely. His Magic Resistance should be high enough to resist anything except archmagi bringing in the heavy guns (wizard's communion, sympathetic and arcane connections, and lots of raw vis). His magic should be able to mimic the Order's most powerful spells. His Penetration should be high enough to affect almost all magi - but low enough to be blocked by a devoted master of Parma. Higher-level (Might 75) creatures should be essentially untouchable by Hermetic magic, and much more powerful than Hermetic magi.
In all these things, Lord Myrsyne's statistics don't fit. His Magic Resistance (50) is so low a master-magus can probably take him down without even needing to pull any of the big guns. His magic can reach respectable levels, but not close to what archmagi can pull off (level 180 spells and more). His penetration is harder to evaluate, but that actually seems better - Parma 10 will block him, and so will lower Parma with some Forms, so that it seems roughly alright.
Bottom line - Lard Marsyne will fall to Hermetic magic too easily, and his magic is too weak in comparison with top-level Hermetic magic. Or in other words - magi's power needs to be brought down, and Marsyne's Magic Resistance needs to be brought up. In my opinion.
Yeah, the spirits can go hide in the Magic Realm and cry for their mamma while we toast their aspects for the evening's BBQ. ... 
Magic should not trounce the mundane, in my opinion. If the mundane forces aren't a threat, the whole setting becomes a farce. All those castles, all those knights - don't really make any sense, and all those histories that fill ArM books are useless.
Leveling cities is something angry gods do, not pissed-off apprentices. And there should really be only very few things with the power to do that. An angry mob should be a concern for most covenants, and an angry bishop even more so. A castle should be respected, an army feared, a crusade should be a nightmare to be avoided at all costs. Wizards should belong in the setting, being at low-enough power level to participate instead of overwhelm it. They can join an army, perhaps advise it; they should not replace an army, or destroy it. They can live in a castle; they should not erect one magically with ease, nor should they destroy their enemy's castle with a wave of their hand instead of a long siege. They may bargain with kings; they should not laugh at their meaningless "knights" and "armies" and ignore those crown-wearing fools.
Magi's power needs to be brought down. Or else - the setting needs to be seriously altered to make more sense; perhaps a High Fantasy setting will fit the current power levels.
Yes it should. I don't care about those castles. If I'm a magus, I only care about where I live, the vis I can acquire, the books I can read, and making sure I have enough silver or wealth to make it happen. It's about the magic. Now if, in a story, the troupe wants to play out what happens when the Mongolian Hordes come to visit, and there's time to prepare, then that's perfectly fine. If the choice is to rain down fire from the sky, doing +5 damage to 100 people at a time, that's going to prove to be pretty ineffective, to be honest. Yeah, one magus is replacing ~100 archers...
Then why are we playing in Mythic Europe ? If kings and knights and castles and so on don't matter, if the only important thing in the setting is the raw vis we can harvest - why not just play in a fantasy setting where we can go and kill monsters and take their stuff/vis ? Forget all those kingdoms mentioned in all those ArM books; they are meaningless.
Look - one can play your way. One pretty much has to, by the RAW. But I think it cheapens the setting and doesn't fit it. The setting works better with weaker magi, and conversely strong magi work better in a more fantastic setting, with powerful supernatural opponents for them to face/hunt.
You're mischaracterizing what I wrote as what I do. It's not my way, but it's not an invalid way. Mythic Europe is not Medieval Europe, it is whatever the troupe wants it to be. If you and yours don't find it believable, adjust it for yourselves, but let's not get into a "you're doing it wrong" argument. And that's what you're doing when you say that Hermetic magic is broken.
Let's go with the premise of the OP based on the RAW and the Setting as Written and not take apart the quibbles you have with 5th Edition. I like that my magi don't have to worry about mundanes in certain settings. In other settings, I will have to worry. Taking this specific spell, most of my characters would be what's the point. Can we buy them off? Bribing armies wasn't uncommon. Can we make our terrain less pleasant to go through? I'm not sure I'd have a PC design a spell that destroys huge swaths of armies, or even damages them. That just makes the PC a target, and how much effort do I need to put forth to destroy the army? One, two, six, eight seasons? Depending my specialty and personality, I'll make a choice that fits, but by and large, it won't be grinding an army into nothingness.
Sorry if I was getting to "You're having wrong, bad fun" territory. It's certainly not my place to tell you - or anyone - how to play, or what to like.
I think that we can agree that if we stick to RAW and sans direct divine (read: SG) intervention, Hermetic magic is powerful enough to obliterate or deflect any mundane army. Whether your magus will want to spend the time to develop the required spells or Arts is another question. Whether you, as group, like that routing armies with magic is a readily available option is yet another question. To each his own.
Yair
I think the point is to ask why would a magus do something. And what response he might expect. And let's be honest, the most dangerous enemies should be other magi! 
By RAW, you total up the ceremony add it to the total. Subtract the number of people in the ceremony. Add each helper's method and characteristic to the total. You can have unlimited helpers. Helpers need not have the requisite method or ceremony, in which case they only add their characteristic. Note that each person with the relevant characteristic of +2 or more adds one or more to the total.
The ability itself isn't really well thought through. If I wander through a random university and get help I can probably find a bunch of people with com or int at +2 or more. Get a hundred people of these people and well... God mode. Its even worse with an group that initiates their people. Get a dozen people with ceremony at 6? Plus 60. Plus their characteristics. Plus any methods they might have.
Most blatantly in the "poorly thought through department" are the ceremonies with leaders. You get no bonus for your ceremony, no penalty for people, but have a maximum number of people equal to the leaders ceremony score. At best, this is the exact same as a leaderless ceremony. (You should probably do something like the infernal ceremony and have a required number of people. Say 13.)
Mongols were the greatest war machine the world has seen since the Romans and before Napoleon.
The Mongol thrust into Hungary was in large part a reconnaissance in force, in part to ascertain the suitability of Hungary as their European base; they certainly did not have the requisite amount of manpower to besiege the fortified points and nor were they particularly determined to do so. When they did, as they did in the Near East or China, castles and forts fell. In any case, like they found in Syria, their foray confirmed to the Mongol high command the unsuitability of both Syria and Hungary as their base.
Russia is an example of where they had (barely) the sufficient ratio of force to area to command obedience, much like the English were able to do in the Welsh Marches in much smaller scale. Really, do you think Russians would have made themselves slaves to the Mongols for couple centuries just because?
Mongol warfare was indeed dependent on horse archery, but not to the degree the previous nomads were. The Mongols knew how to use infantry as well and they had excellent commissariat. Both the Song Chinese and Koryo Koreans practiced scorched earth policy against the Mongols (with varying intensity in case of the Chinese) and the Mongols showed themselves equal of Caesar with methodical, brutish, unglamorous campaigns of attrition that ground those countries down (moreso in Korea where there was both national and political will to resist to very nearly the bitter end).
And if you throw in the magi, why not for the Mongols? In this case, YMMV, I find it absolutely ridiculous that the Order of Hermes should be better than anything the Far East can throw. In my game, Mongols would be aided by mystics or magicians at the very least equal to the Order at least in firepower. IMO, that should be way to go, or else you're just juxtaposing the 19th century Western dominance onto 13th century Eurasia, when the Song China was in fact the most advanced civilization in the world (it'd be all downhill for the Chinese after the brilliant Song zenith, after all). (In my game, I'd make Buddhists and Hindus Divine, and I'd make up a Tibetan Buddhist sect specialized in detecting and destroying magical phenomena. This would explain both why the Mongols would need an army and why Chinese magi couldn't save their land.)
You know, if you have the battle of Mohi go the other way, why not have Battle of Las Navas de Tolosa go the other way? Instead of Christian reconquest, you have instead Muslim reconquest. Or how about the Ayyubid navy defeating the Italians and taking over Cyprus and the Levant few decades (centuries for Cyprus) early?
If you want the Mongols to be a threat, just make them infernal or divine (IIRC those were the theories back in the day) and grant the basic trooper MR30 and MR up to 75 for the high end stuff. Suddenly magi might not get so cocky around them.
MR 50 is nothing really serious, though. A competent middle aged magus that has had some combat practice can beat that without arcane or sympathetic connections on a regular basis. At least in the sagas we have played. What is difficult is to deafeat a horde of MR50 creatures, but one of them only tends to be a "who shots first" situation with hermetics.
But if they do that and go around and kick out the Mongols from their backyard (like Timothy said) I can see the OoH facing a crusade against them, since they will have shown their true combat potential, and the other powers of ME will be really fearful of them. NOt the best situation for the OoH, really, even if they win.
Xavi
Source? Even were it true, it still doesn't make them any good at taking stone fortifications, at which they were profoundly unsuccessful in the Hungarian theatre.
That's just false. The Mongols knew the land was perfectly suitable as a base and offered a Mongol bride to the King of Hungary if he'd allow them passage into the country, to use as a base for attacks on Italy, before the invasion.
...and yet far smaller German armies were regularly successful in besieging places which the Mongols failed to take.
...and yet Attila found it a perfectly suitable base, as did the Arpads, and both used similar styles of horse warfare.
No, as I said they maintained their power by using local force.
...and yet none of this speaks to their inability to take fortifications in Hungary, which fell quite easily to the Turks slightly later, and the Greeks slightly before.
You could, but in the standard setting, the Order are the finest magicians on Earth. That's their core premise.
I think the secular nature of Hermetic magic gives it great advantages over, say Taoist magic.
I would be doing no such thing. If you ever do run the game, perhaps you'll find my articles on modelling Song-era magic in Ars Magica useful. styren.pagesperso-orange.fr/herm ... ermes1.htm. The articles predate The Mysteries, but some of the ideas still work. They are in issues 10 and 11.
Weird to make the Tibetans your sock puppets when they are basically the source of all of the Jedi / Sith stuff in Star Wars, and so can hold their own with the Diamond Vehicle, but there you go. You obviously disagree.
I'd be quite happy for Muslims to take over early post-1220 in one of my games. The point from which historical diversion occurs in vanilla games is 1220, so Los Navas de Tolosa is kind of set unless there's an earlier start date in that game. If there was, I'd be entirely happy for it to go the other way. I mean, in one of my games the player characters turned up during the Normandy Landings in thr Second World War, and in another they discovered Stargates and fought aliens, so the idea of the Muslims winning hardly phases me, frankly.
Your sources to the contrary?
It's the absolute truth. What you're referring is before the invasion; post-invasion, they found out that the Hungarian Plains was actually inadequate to provide for their mounts. Nobody knows why the Mongols left so abruptly, but the most plausible one to me appears to be the inadequacy of Hungary as a major logistical base.
What are your sources for claiming that the Mongols tried to besiege Hungarian fortifications? Because none of my sources say that. They say that the Mongols spent the time garrisoning, foraging, and rounding up the civilians. I've got nothing on them besieging forts.
German armies? Are you talking about after the Mongols, or before? Because Hungary did not have very many forts pre-Mongols.
In any case, Mongols weren't in Europe long enough to engage in sieges. They were in Hungary for less than a year and during that time, their armies were dispersed throughout Hungary. Nor have I read anything to indicate that the Mongols had a siege train, which reinforces the reconnaissance in force thesis. Compare that to the 2nd Mongol campaign in the Middle East, where they had a siege train and took cities that deviled both the Crusaders and the Muslims for years. In contrast, the 1st campaign was like the invasion of Europe, where fortified points were bypassed rather than besieged.
Magyars had nowhere near the numbers that the Mongol armies had; otherwise, Magyars would've made a far more impression on Europe and state-building in Germany far more advanced to cope with such numbers. And Atilla? Apples and oranges. His armies were widely dispersed all across Europe, with only his core force permanently stationed in Hungary. His numbers were inflated with vassals, who were all over Europe.
And that was because the Mongols had sufficient number of men to cow the Russians. Had they not, the Russians would've thrown off the Mongol yoke much earlier.
Was it inability or unwillingness? At the very least, you have to show that they tried and failed to take forts in Hungary, as opposed to merely bypassing them. As for Manuel, my understanding is that prevalent castellanies at the time of Manuel were predominantly of earth and wood type, not that difficult for the sophisticated Byzantine siegecraft. It was during Bela III that stonework became common and a few cities were walled in masonry. And the Turks? Well, they're a bit out of range of my expertise, but I believe that by the time they invaded Hungary in earnest, they had cannons.
Where does it say that? Finest in Mythic Europe, yes, but the finest on Earth?
Just a general comment about your post, and then a specific comment. It seems awfully dodgy to turn around and ask someone else about their sources before providing your own when asked, first.
Yes. The nature of the game is that magi in the Order of Hermes are the finest, most powerful magi on Earth. And Earth is a pretty small place, past the middle east are the lands of the fae. That's the nature of Mythic Europe...
Because it was a dodgy question to begin with? Ask ten military historians who was the greatest and you'll get ten different answers...
It's a fair question for someone to ask when they know something to the contrary.
How is it fair?
And does he know "something" to the contrary? After all, he then continued with "even were it true..."