USSR

I still think that the worst thing to happen to the West in my lifetime was the fall of the Soviet Union.  We knew the bad guy, but we didn't fight.  Our side prospered, and we were made to prosper to prevent the spread of socialism.  Terrorism wasn't tolerated, partly in fear if what it might provoke.  Each side kept their rogue states and baddies under control.  Mutually Assured Destruction worked pretty good.