Originally Posted by
Ken Cheng
It wasn't just the Sung Dynasty; "using barbarians to fight barbarians" has been in the Chinese (and probably other civilizations') playbook since time immemorial. It often backfires, especially in the long run, but it has also been used successfully at times. It's always kind dicey, however, because there's no telling how it will play out.
U.S. President Ronald Reagan played this game with Iran and Iraq during the 1980s. To some extent, Hitler and Stalin also played this card during World War II in terms with how the Nazi and Soviet regimes related to each other and the Western Allies.