Create the least morally ambiguous nation or conflict possible. As in, basically everyone not involved sees one side as being the "good guys" and the others as the "bad guys".
Even WWII, which had two to three arguably 'evil empires' (Nazis, Japan, Soviets), was not this morally ambiguous because the Allies were, though democracies and enemies of the Axis, oppressive to their colonies and minorities. And the Soviets were on the Allied side.
North Korea approaches this situation closest but they still have China as an ally, and the North Korean people are seen as victims of their oppressive and totalitarian government.
The Nazis alone, also, can be seen as very obviously the "bad guys". But not all the German people supported the regime, and many non-Germans filled the ranks of the Nazi army.