Any system of beliefs is going to have a few internal inconsistencies. Speaking from cheerful ignorance of what Buddhists actually believe, from my understanding the "don't kill things" is something in the nature of really good advice grafted on top of a philosophy that really an acknowledgement of impermanence and a set of guidelines around how to live with that. There is no sustainable belief system that stops its people from becoming soldiers.

If everything dies, at some level killing isn't such a sin. There are real practical populations if people use that as a serious philosophical starting point though.