We've had a lot of time to squirrel away resources for this. 
Squirrel away? I don't think I've ever completed a week without spending at least 75% of the gold I got the tax before. Most of the time I made sure to spend a little more just before tax day so that I didn't have to pay taxes...
Thalmarkin has been on one big spending spree since the end of the invasion and it's all been in preparation for war. Even the tournament we threw (and that ended up costing us only 190 gold) was a nice recruitment campaign for new nobles.
But yes, all that spending got us a pretty decent army.
