I came of age during the Vietnam war. I was not a JW then, but in my mind I couldn't justify going to fight there. I would have tried for conscientious objector status, had the necessity arisen. It didn't. For three years I was eligible for the draft. They didn't call me, and I didn't volunteer.
Since then, I've never had to consider the possibility of fighting in a war.
Now, I can tell you this: I will defend what's mine. And I'm quite capable of killing another person who threatens me or mine. I would go to fight against armies invading my country. Whether I would be willing to leave my country to fight on foreign soil would depend on how convinced I was that such action was truly a necessary defense of what is mine.
COMF
Edited to add this PS:
When I say "my country," I don't mean that I am patriotic to the United States simply because it is the United States. I mean that something threatens my home, and I will fight to protect my home.