In Lords of Men, generals do opposed contest of Area Lore + Int, nothing else. There is no "tactics" or profession: General, skill.
In a way, this makes sense (if you know the land, you can more easily exploit it to your advantage), but it feels as if something is missing: the many victories of Mongol armies, using clever stratagems, make little sense, unless you consider they always had the weight of numbers on their side
So, aside from just getting the highest intelligence possible, how would you create a general or tactician?
A high score in Leadership is nearly a must for a general.
I would also consider good scores in Awareness and Guile. (To notice enemy tricks, and to come up with clever ways of tricking the enemy.)
Folk Ken and Animal Handling to understand how people and animals (in your own force and the enemies troops) are likely to react to various situations, and what can be used to persuade them.
Premonitions would of course be useful, but then we are heading away from a purely mundane general and into supernatural stuff.
I looked into this, and actually there's a whole host of areas to military success using Lords of Men. Raising troops is Presence + Leadership, and having the Inspirational virtue helps gain allies. To get the Deployment total (which The Fixer alluded to in their original post) you don't have to roll it yourself - you can appoint a lieutenant, so maybe Social Contacts: Scouts would be useful (you have a network of local guides who can help with local knowledge, and this would also help when generally adventuring in Ars Magica - useful for a companion to have).
Battlefield events use Stamina + Leadership.
So, using the LoM rules you want lots of leadership, improved characteristics and maybe Great Intelligence or Great Presence, and Social Contacts: Scouts. Puissant Intrigue may help you scheme to know when people are plotting against you, and help in gaining allies - it certainly seems to have been the main skill used by the leaders of the Third and Fourth crusades.
Both would need some Profession: Soldier and a lot of political skills. The latter to assume and keep a position as "general", the former to know what specific soldiers could and would do.
Area Lore: Battlefield might also be useful - but in 13th century battles were rare, and meaningful Lore about them hard to acquire.
If you're being cheeky, and wish to test the boundaries of what your troupe, try seeing if you can have Ways of the Battlefield - justify it as a terrain type (after all, City & Gild allows merchants to get Ways of the Road or Ways of the River) and get the useful bonus to dealing with soldiers (inhabitants of the battlefield) and reducing botch dice by one.
You could not make rules to cover their kind of success. All the really great military successes break the rules of the previous wars. Whatever mechanics we decide on, the best strategy is to come up with a novel idea which does not fit.
I agree with all the other posters, each proposing some skill or mechanic which may be relevant for some approach. The truly successful general with come up with something that surprises the opponent, surprises the SG, and does not fit the rules.
Additionally different tactics will require different skill. Leadership is always going to be a factor, but beyond that there is a huge difference between leading a successful siege through constructing catapults on site, through bribing guards to open the city gates at night (siege of Jerusalem) and leading high mobility horse archers in a series of surprise raids which can only be accomplished because the wagon train logistical infrastructure has been replaced with herds of sheep providing food and supplies and having a diffuse army (to cover acreage of grass) which converges on tactical points (Mongol invasion).
I use two rolls, which I thought was from LoM, but maybe it's an accidental house rule:
Opposed Intelligence + Area Lore for the strategic side of things - the winner applies the positive difference to the next roll:
Opposed Presence + Leadership + (advantage) for tactical side and to lead the men into battle and coordinate forces
Botches are catastrophic.
Medieval battles were considered very random & risky. Even with great number disparities and many advantages things went wrong and more powerful armies were defeated.
As said above, there are a whole host of other things skilled generals did. The level of complexity I have is what my troupe wants -- even for a military campaign.
If botches are catastrophic, then Cautious with Leadership may be a good pick, as it both increases your chances of not losing horribly on the battlefield, and when your strategist tries to throw his weight around by using Leadership for intimidation it may prevent horrible embarassment.
