Since murder is against God’s Word, is it a sin to fight in a war?
It’s a tricky question. God has sent His people into wars to attack and kill people He described as His enemies. He could not ask men to sin on His behalf. I guess the question is whether a particular war is being fought because God has ordained it, or because leaders of nations have declared it so.
Paul wrote that as believers we’re to obey our leaders, because they were put in place by God. (Romans 13:1-7) I believe that makes them accountable to Him. To me that passage exonerates the soldiers, but not the leaders, in the case of a war that God has not ordained.