The church of Jesus Christ is larger than any one nation. It's members live in every country of the world and are called upon to represent the kingdom of God to those nations. Like an embassy that supports only the agenda of its homeland, so the church supports God's agenda alone and not a nationalistic agenda of the country where it is located.
When nations wreck havoc and engage in mass destruction, such actions serve the purposes of Satan, the god of this world.
Therefore, when one nation declares war against another nation, the church cannot support that action. Nor should it encourage its members to fight in the war, since that would involve Christians from one nation killing Christians from the opposing nation. The church must not place loyalty to a nation above loyalty to the kingdom of God.
What should the church do? First, call upon its governmental leaders to seek non-violent solutions. Second, if war is declared, it must state publicly and unequivocally that it opposes the war. Third, it should weep for the victims of war and minister to them and their families. Fourth, it should preach the gospel of the kingdom and call the nation to repent and submit to Jesus as Lord.
The church must demonstrate what it is like to live under the reign of God. In presenting an alternative path to peace, the church reflects in part what the rule of God will look like when the kingdom arrives in it's fulness.
When the church embraces the ethics of a nation, it loses it's prophetic voice and it's identity as an embassy of the kingdom within that nation. It becomes just another organization that supports nationalistic causes, rather than a vehicle that calls people to align with the kingdom of God.