Alternate WW2: Quick German Defeat, Allies Attack Soviets

In OTL, Churchill advocated the idea of the Allies attacking the USSR once they defeated Nazi Germany (Operation Unthinkable). However, the plan was deemed unfeasible (understandably, due to Soviet manpower and Allied war exhaustion) and thus was not executed.
But what if Nazi Germany's offensive was curbed early on in the war, before 1941 (how this would happen is open for debate) and the Allies subsequently attacked Soviet Russia?
Is this a feasible scenario or would the Allies just settle things diplomatically? If the former, what would the outcome be: Could the Allies defeat Soviet Russia, or would the war become a meatgrinder like during Operation Barbarossa? If latter, how would TTL post-WWII world look like? Also, how would this change the Pacific theater since Japan only provokes America to war in OTL December 1941?
 
Britain and France aren't going to attack the Soviets if Germany is defeated in 1940 or 41 unless Stalin joins the fighting on Hitlers side. While East Poland and the Baltic States are problems, they're not worth what an attack on the Soviets would involve. The Soviets will be shunned and sanctions imposed but that's all. I know there was talkofsendi forces to fight in Finland but in the Winter war the Soviets were seen as German allies, and it was an excuse to "secure" Norway and the Swedish iron ore deposits.
 
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