In the past, a world war has been one consuming most of Europe

Thus there will probably never be anything defined as a world war anymore. The EU keeps Europe united and the UN keeps the world united. With most nations being or progressing towards being democracies there will most likely not be any huge disagreements that could split apart those unions.

This "war" won't be considered a world war because it's practically one sided. Everyone except a tiny band agree that terrorism is destible and there won't be much of a war. It'll probably just be a large scale UN police action, if even that.