Generally, war was a huge gamble for a king because mustering and provisioning armies required so much money that most had to go into debt/borrow. The only way war was cost-effective was through victory. Fiat currency didn't exist yet either so the economic ramifications were felt very swiftly at home so wars also had to be over quickly. Defeat not only imperiled the life of a king because he was usually on the battlefield, but also because his tax and human capital base would be exhausted and his capital city would most likely be incapable of defending itself.
I'm terribly sorry, but we prefer questions to have a bit more specificity in regards to place and time. Feel free to resubmit this is you have one in mind (or more than one and make multiple submissions), ie. Medieval Europe etc.