Well, WW2 did involve Christian nations fighting each other. The Vatican was located in the Fascist state of Italy.
So did WWI, and most if not all of the European wars including the 7yrs. war which helped shape future America were over Christianity or between Christians.
Not going to go into big history here but if you are interested some contend the subject you mentioned started with the "Roman question" (temporal power of Popes), and that the Lateran Pacts between Fascist Italy and the Holy See ended it. There were also agreements with Napoleon, Concordat of 1801, and the "Reichskonkordat" with Germany 1933 (The Reichskonkordat is the most controversial of several concordats between Germany and other nations that the Vatican negotiated during the pontificate of Pius XI.)
https://en.wikipedia.org/wiki/Reichskonkordat
Jesus said his kingdom was not of this world yet much of the what we've seen from most is trying to establish God's kingdom here so they can run it or use the church as moral backing and public motivation for conquest.
Quite a photoshopped pic there...