Especially when their churches suggest them like what they had done in German during WWII
Defend the righteous, protect the weak.
The church should not become the tool for the politics like German Christians in WWII.
For a war and the countries involved, the leaders are very important. Christians should pray for them to have fear before God and fight for the righteousness.
But what should the church do?
Paul said in Romans “Everyone must submit himself to the governing authorities” and Martin Luther thought even though the government is evil, it just means that they are the executioners of God.
If I was in such a evil war, I will quit. I will pray to God to stop the war.