Actually I'd go with a different version of worst: the USSR takes a different military tactic and decides to postpone the Purge until after its armies trounce the Nazis, leading to Tuchachevsky and Blyker and their ilk preparing, executing, and organizing a massive, successful defeat first of Barbarossa and second overrunning much of Central and Eastern Europe in a staggered, unfolding process with an army that's the most modern and powerful of its time, this before the USA even gets to its Midway point in the Pacific War. Instead of cocky, hubristically stupid Germany, or cocky, idiotic US hubris, the world sees the already-messianic and brutal Soviet regime replace the Nazi enemy with a new enemy that reclaims almost all of its 1914 borders and is a nearly-unstoppable force, militarily speaking.