But to be fair, I don't know how brave Ulfric would be (or any Nords) if they were unequivocally beaten and facing genocide, or a grudging and temporary phase of cooperatoion. They might do the same thing (or not have a country). But I feel the imperials care a lot more for other race's than Ulfric's lot.
And building an army to be capable of fighting a threat is not the same as building an army with the full intention of taking them on. It's common practice for any nation to sustain a military they feel is practical to defend themselves. It doesn't mean they will use it to invade another countries enemy (unless they themselevs are attacked, which guessing from context, could easily be to what he's referring). Ulfric's track record of helping other races is pretty poor, in case you haven't noticed.
