There are indeed just grounds for war according to Romans 13:1-4, 2nd Peter 2:13-14, and Psalm 82:3-4. In a Christian nation particularly, a war to deliver the oppressed from the wicked, for example, would be a just war. It's harder for a nation made up of Christians and Muslims and Jews and atheists, and everything in between to decide what is wicked and who is oppressed by whom.

To my knowledge there are no Christian nations on the Earth, and those which do have an official national church do not govern their own citizens accordingly, much less reflect Christian principles in their foreign policy.

-Robin