Teribus,

The most important thing is the welfare of your citizens. This is far more important than whether you win or not. You can argue in some cases, such as the war with Nazi Germany, or a possible war with ISIS (I am still far convinced that there is such a war) that your citizens welfare would suffer so much in the event of defeat that victory would be imperative. In other cases, not so, I would argue that WWI was a case where it was not so.