When I got involved in the peace movement, back in the early 1980s in Germany, there was a sticker that was very popular. It read "Imagine there is a war and nobody joins"
Of course, this is very naive, but still the idea is important, and it also it points to the important fact that every war is fought by people - human beings who could also make a different decision and "not join". My organisation, War Resisters' International, was founded back in 1921 to support people who don't want to join a war - conscientious objectors, war resisters, and deserters. But "not joining" goes beyond the mere refusal to fight, to join the military. War means that an entire society is organised to go to war - it is a mindset, and it includes participation on all levels of society.