Military leaders don't decide to go to war. That's for elected leaders. Waging war and studying how to wage war are the proper roles of the military. That said, any civilian leader with a clue should ask their military commanders what a war would entail, those critical "how much" questions. And if the numbers don't add up, probably take a pass, look for a diplomatic solution. This is where honest warriors like Shinseki lose their jobs and people like McNamara, Taylor, and Westmoreland end up in charge.

As for reading up on the general principles upon which the Western world built its world order, how to know when a war would be just, the collected works of Plato, starting with The Republic, and the stoics would be a good start. I would advise Irvine's Guide to the Good Life as an introduction to the stoics.

I would also add Errol Morris's The Fog of War, including McNamara's separate 10 lessons (1), and, as counterpoint, Ken Burns' Vietnam series. The two together are an unparalleled view of the entry into war, and its consequences.

(1) https://en.wikipedia.org/wiki/The_Fog_of_War#Ten_additional_...