Why do JW's believe war is wrong?

As a Christian I do believe some war is wrong, but how can JW's believe that all war in itself is wrong when God called his own people to war in the past? I just came across Judges 3:1-2 "These are the nations the LORD left to test all those Israelites who had not expereienced any of the wars in Canaan (he did this only to teach warfare to the decendants of the Israelites who had not had previous battle experience):" I get that there is a difference between the old and new testaments when Christ came and fulfilled the law; but surely God himself doesnt change, and if war was evil in itself, it would have been wrong for the Israelites, if indeed it's wrong for us today.