I've never been in the military, but I've spoken with some who have (Viet Nam) and read accounts by others, from many different wars. Once you come right down to it, very few soldiers are fighting for some nebulous "cause" dreamed up by politicians and generals who are safe behind the lines. They overwhelmingly state that they are fighting for their buddies, the guys next to them in the foxhole, or the tank, or the plane. There sole motivation, besides saving their own lives, is saving the lives of those fighting with them.

Here http://en.wikipedia.org/wiki/Christmas_truce is a story I remembered about an occurrence in WW1. Supposedly, this shocked and dismayed the politicians and higher officers who feared it could demoralize troops and bring an early end to hostilities! To me, it points out the problems of the common soldier, forced to fight for a cause which he may or may not believe in, which may or may not make for a better life for him and his family, against other men who feel exactly the same way. And the ones who benefit the most are the one's least likely to be harmed.