IMO,the war required three powers to win. Britain bought time,the U.S brought supplies and the Soviets bled the Nazi war machine to death. Removing one element destroys the equation. For example,lets assume the Soviets are knocked out in 1941 through the capture of Moscow and Stalins death/execution.The Soviet union is reduced to a rump state near the Urals. The Germans control European Russia with its grain and oil supplies and are in a good position to thrust down through the Caucasus to threaten the Middle east and British India. So,where does that leave the U.S and Britain? Well,not in a very good position. The Germans will throw everything at their remaining enemies. The blitz is renewed and the R.A.F will be put under fierce strain. U.S planes and pilots will ease the burden on the brits,essentially its an attritional battle. The allied strategic bombing campaign goes out the window as the focus switches to defending british cities from the luftwaffe bombing. This probably continues for about a year,with peaks and periods of little activity. Eventually,the Germans will realise that as long as the U.S is backing Britain it can't be directly invaded and knocked out of the war. The Kriegsmarine is able to churn out more U-boats and rationing becomes tighter in Britain as more Merchant shipping is sunk.Eventually,German jets,rockets and advanced U-boats will be produced in sufficient numbers to strangle/bombard britain into surrender. If Britain is able to hold out until 1945,the U.S A-bomb will change the situation. U.S can't drop the A-bomb on a German city due very strong luftwaffe presence and the danger of an unexploded bomb being captured by the Nazis.Bomb is detonated elsewhere. Announced to world,tech is shared with britain. Germans propose immediate peace treaty,which is accepted. Cold war ensues.