An empty treasury
Troops expect to be paid. Run out of money and they lose heart.
If your War Budget ends an hour almost empty, even after the hour's income comes in, your troops go unpaid. Their morale and loyalty start to fall, and your people grow unhappy too. Your advisor and the War Budget warn you before it happens.
Leave them unpaid for too long and tired troops stop taking orders. They still defend themselves, but they will not move. The worst may desert, or even change sides.
As soon as money comes in again, morale starts to recover.