Allow me to present two counterfactuals that may have resulted in knocking Britain out of the war early, and consequently the United States.
They are rooted in decisions taken within the German high command in 1940, mainly with Goering. The first - Field Marshal von Rundstedt does not issue the “Halt Order” allowing the British Expeditionary Force to evacuate from Dunkirk. Goering does not influence the Fuhrer on the decision and the vast majority of the BEF along with French elements are captured.
The second - Goering orders the Luftwaffe not to engage Fighter Command over the skies of Britain, instead allowing the British unmolested on their island. There is no ‘finest hour’; no morale boosting boys repulsing the Luftwaffe in their Hurricanes at Spitfires. The writer Max Hastings explains better than I could the importance of ‘finest hours’ and how Churchill was a master of utilising them;
In the face of the calamity at Dunkirk and with no display of Britain’s fighting spirit against the Germans, pressure mounts on Churchill as to just why the British should stand along against a Germany which is already master of the continent. Following the disastrous Norway campaign, the catastrophe at Dunkirk and no Battle of Britain, another vote of no confidence is called in the House of Commons. Churchill is forced to resign.
Hitler, never wanting war with Britain in the first place (in Mein Kampf he speaks of an alliance with his fellow Aryans), sends out peace feelers which are immediately accepted by the new British government. A peace is established on the basis that Britain will not interfere with German efforts on the continent if German does not interfere with British Empire affairs. The men captured at Dunkirk are repatriated and the public gladly close the book on a brief but inglorious period in their history.
Consequently, there is no Battle of the Atlantic. U-boat operations never harass United States shipping. With Britain now at peace, lend-lease does not exist. Isolationist sentiment is strong - there is little will to become involved in a war in Europe that has little to do with the American public. Many Americans even admire the industrious German triumph.
So, in 1941 on the eve of Operation Barbarossa the USSR stands ready to take the greatest sucker punch in history - but with no prospects of any western aid. Can the Soviets still drive the Nazi war machine out of the Union and push the Wehrmacht back to Berlin, or is Stalin doomed without the western Allies?