I see that nobody chose changing. If I had a second choice that would be my word. I think that World War Two changed warfare forever. "If you had a second word having nothing to do with your first what would it be"
re changing but not in meaning of warfare: all political world map changed. Communism came to a power that no one would dream of before and British Empire falled, and America would not grow so strong if not the war, and IMO it would still remain isolated in a way + concerned of its own business much longer.