They'll get swamped by hordes of nomads. Quantity, my friends, quantity.
That's the point of guerrilla warfare though. To not get swamped and pick choose battles because direct confrontation is death.
Comrades, IRL the mongols were nigh impossible to beat directly.
The best way to hold them back was a lot of fortified locations using area denial and ambush/raid tactics aka gorilla warfare.
You'd think that having more troops would help you, but it really wouldn't. The enemy has numbers, more importantly, trained numbers, all of which on horseback. Even if we could recruit to have an equal number of soldiers, they wouldn't be as trained, nor would they be on horseback. Furthermore, they'd only be mostly useful for direct confrontation and would probably screw up a lot of raids.
Small trained units would result in the best case scenario for dealing damage to the enemy. We hold ground where we can, and focus on grinding the enemy with a higher k/d ratio to get rid of their numbers advantage.
If we can A. Force them down to numbers closer to ours, we would win the war most likely.
or if we can B. Kill their hero.
Therefore, If we focus on elite units that maximize damage dealt while limiting damage taken, we can attrition the enemy into defeat and force them to end their incursion. If we kill their hero, our martial hero(es?) can make due and end the nomads, as they will have lost their head, their unifying factor. If they really are an expy of the mongols, they might even return home for the funeral.
Also, question
@Oshha When you say the nomad incursion began fornicating,
did they do any of it with our people? Or was it just them mating with each other while others worked in forced labor?