Yes, any law, not even just religion, without war plans is incomplete. The world is not a colorful peaceful place to live without having a strength to protect. War is not necessarily about oppression, it is about defense too. Some people never tasted fear like in other countries plagued with war to know that defense to protect also requires warfare preparations. And no, it is not something to be taught to and done by everyone of course, but specific trained personnel chosen to be on the protection side. In the past many countries had their people enslaved, resources stolen and land overtaken because their warfare tactics were so weak or maybe non existent.
Christians don't do it yet let their govt do it? What difference does it make if they "let" "their" govt do it? Also, I don't think all armies, American for example, have 0% Christians, there must be at least some. Also, it is not like all Muslims do it. I'm Muslim and I don't do it, as well as millions of other Muslims.