It is a doctrine that goes back to the end of WW2. The Western world began to see what Churchill saw early on, that Russia was as big a threat as Germany had been. People forget that Russia invaded Poland at the same time as Germany from their border with Poland. If not for Hitler's duplicity, Russia may well have fought on Germany's side.
America chose to be the world's policeman. No one demanded it. No one forced it on America. No one forced America to attack North Korea, Vietnam, Afghanistan or Iraq twice.