The Battle of Britain

True, the Germans would still have been in 'over their heads' against the US and the Soviet Union.  However, there are a number of important issues here that could make a big difference.  If Britain is assumed to be 'defeated', does that simply mean that the British Isles have been 'neutralized' or 'occupied', but Britain and the Commonwealth are still in the war?  Or does one assume that Britain is out of the war.  Without Britain (and Commonwealth) actually being at war, does Hitler even declare war on the US - after all the 'assistance' the US was giving Britain and the resulting 'undeclared' war in the Atlantic were major considerations in Hitler's decision to declare war on the US.  Furthermore, although the US clearly had massive potential, there was a signficant lag before the US was able to apply that potential to the battlefield in the ETO.  Although the US was 'openly' at war with Germany in Dec. 1941, and in spite of the 'official' adoption of the 'Europe First' strategy, it wasn't until Nov. 1942 that American troops joined in the land fighting in a significant way, with the Torch landings.  Even then, the British played a major role in supporting / facilitating the Americans in the landings - with their assets in the Med in particular.  Without the British, the Americans would have been much further behind the historical schedule.  Frankly it is difficult to see how they would effectively intervened in Europe with Britain completely out of the war.  With a large delay in the US projecting force into Europe, Germany would have been 'one-on-one' with the Soviets for a much longer period of time.  The 'front' against the US and Britain played critical roles at 2 'turning points' in the war in the east.  First, during the Stalingrad period the Germans were using critical air assets to transport and supply the forces necessary to set up a front in Tunisia.  Then the offensive at Kursk was called off due to the landings in Italy.  Without having to fight the British and later American forces, the Germans would have been able to attack the Soviets a bit early, with a bit more forces and would have been able to concentrate on the eastern front for a bit longer.  The absence of the historical bombing offensive against Germany would have freed the Luftwaffe to provide greater air support against the Soviets.  Still, it is difficult to know for certain, even given all that, whether or not Germany could have defeat the Soviet Union.  Even a 'victory' in the terms defined by Hitler - i.e. the A-A line, would have left a 'hostile' rump state east of the Urals.  However, clearly such a scenario would have been sufficiently different from the historical to justify the conclusion that Britain remaining in the war, with the British Isles still 'free' had a huge impact on the course of the war, and determined the outcome to a significant degree. 

There was a narrow window of opportunity wherein the Germans might have had some chance of success albeit very slim.

That period was between the fall of France and september. They would still have had to defeat utterly the RAF (not possible) and be able to contend with the RN.

After that weather and growing British strength would make it impossible