Hitler is murdered by an unknown individual upon leaving Prison, resulting in a continuously fractuous and weak Weimar regime and a Germany that remains disarmed.
However this leaves Stalin unchallenged in his domination of Eastern Europe, and in the 1950s a fully industrialised Soviet Army marches into an unprepared and divided Europe that is too busy squabbling with itself to unite and coordinate against the Russian giant until it is too late.
Aided by local socialist insurgents that have been fueled by an even longer Great Depression, the Soviet Army marches all the way into France, and soon it looks like the British may be the last free peoples in Europe, with the US unwilling to help following a casualty heavy war with Japan in the Pacific.
But the European Allies rally, eventually smashing the Soviet War machine.
One of the most plausible ways right there. You also have the Holodomor and Doctors Plot, with no Holocaust on the other side, and with the US not allied with Russia, we don't have to gloss over Stalin's crimes.