I've always had thoughts that leaned towards pacifism but they never really came into fruition until I joined the military because I agreed with those around me that although the Bible seems to point to it, it just can't be true because it doesn't match with what the flesh wants....and it's just too impractical.
About two years after I joined the military I had a good idea about the philosophies that are running around here but since I am only in one branch, I started to dig further. I started to talk to infantry guys, vets, and other Christians to see their thoughts on the military. Actually I started a thread on here not too long ago on the subject. The most learned man on the subject on that thread was a pacifist and nobody could debunk his points.
At that point, I was persuaded but I kept digging. I started bringing up his point to other people and examining them myself to see if they checked out. They did and others could not debunk the points that were brought up either (which were historical and biblical).
Furthermore, I started examining the involvement of different Christians of different nations in war (Nazi Germany, China, Palestine vs Israel). I realized further that it didn't make sense. I realized that many Christians had their own ideas about the boundaries in violence and war but they seemed to be contrived. Because no one wanted to come to Christ's standard, most were in a state of compromise in which they made up their own standard of when it is right and when it is wrong to proceed with violence (i.e. if it is a "just" war, if someone else is threatened but not me, if it is not persecution for being a Christian). I realized that these standards do not come from the Bible but from man and from culture.
There are several other things I studied, such as the early church's view on war as well but all these things only contributed towards a negation of the supposed biblical command to defend the innocent with violent means.
My point though is that although I cannot I say I was completely unbiased, at this point I was willing to do whatever the Lord was speaking into my heart. So if someone had presented evidence that tore up my argument or called it into question, I truly would have considered what they were saying and likely would have chosen to stay in the military as a light to those around me.
Many cannot say that they will do whatever it takes if shown, via Scripture and history, that their current philosophy is moot. They will stick to the same course they always have because it is easiest and doesn't conflict with the culture around them. Trusting God is hard, especially when you are living in sin. For example, ask a lukewarm Christian to sell all their stuff and move to Egypt and preach the Gospel. Even if God came down in a pillar of fire and did this, he would probably convince himself that it couldn't have been God because God would want him to do etc. etc. good things that cannot be done if he moves to Egypt or if he sells all his stuff.
I'm not saying everyone who isn't a pacifist is living in sin but we all should be questioning our own motives and what if there is something that isn't entirely pure behind our motives to not change our views.