I beg to differ. Operation Barbarossa was the one turning point of the USSR invasion. Stalingrad was merely the consequence IMO as Barbarossa had led to German army being both weakened and stretched too far out. They simply could not compare to the Soviet war machine's recover-ability and production.

Of course Pearl Harbour was what got the US directly involved and nuking Japan out in the end, but I hold the speculation that without Pearl Harbour, US would still declare war to either Germany or Japan anyway because of several reasons below: 1) US businesses were being harmed by the Axis, i.e. Germans attacking cargo ships meant to supply arms to Britain. Sooner or later the piled up losses would justify the entry in front of Congress.

2) The Japanese was expanding fast in Asia and they would not stop just short of US territorial waters, plus the imposed embargo had been pissing the Japanese off anyway. A clash was inevitable.

3) Letting USSR being the major player and eventual winner meant letting communism spread throughout Europe. There had to be a sizable participation from the Capitalist group in the war and the rest of the Western bloc was too tattered to muster that up.