Did you know Britain may have actually saved the USA. You Americans think you saved England and France but forget it was the British who halted Hitler in the west, first by managing to fight off Germany's attempt to invade them. Had Britain lost the Battle of Britain the USA would have had no place to launch their efforts to defeat Germany. They could never have launched an invasion without a safe place like Britain to prepare for an invasion from.
Did you know Germany was working on atomic weapons well before the Americans started the Manhattan project. The British found out and trained a group of Norwegian commandos to go into Norway and destroy the heavy water plant that was producing heavy water for their work developing an atomic bomb. Imagine if Britain had surrendered and this plant and the German atomic bomb program had gone ahead. Without Britain free to do something about it likely the USA would not have known about it until bombers flew across the Atlantic and started to drop them on cities like New York, Washington, and other east sea board cities of the USA. It could have been a squadron of Luftwaffe planes all split up and take a city each and the US cities with no air protection would have not been able to stop them, and it would have virtually taken the USA out of the war in one day. Yes Germany had planes that could fly the Atlantic at least one way and do this. Imagine the Germans capturing neutral ships and putting an atomic bomb in the hold and sailing them into American harbours like San Francisco, Seattle, New York and many others to explode them. Imagine if they had set one off in Pearl Harbour and caught the American air craft carriers there and wiped them out. It would have been the end of America's fight on Japan. It was a very possible scenario which thankfully Britain was able to put a stop to.
Britain was also able to stop the Germans and Italians from controlling the Mediterranean sea so Germany could not get to the Middle Eastern oil fields. Yes America was in the war by the end of that campaign and did send armaments and some men to help but largely it was the British and the Australians that held the Germans off for nearly 2 years before the US did anything to help. If Germany had got those Middle Eastern oil fields it would have made them much more capable in the war but struggling for oil helped slow the Germans down.
Then there was the breaking of the Enigma Codes the German secret coding machine they believed was undecodable by the allies. The British managed to work out how to break the code and it is believed doing that and knowing what the Germans were doing cut the length of the war by 2 years.
All of these things were largely done while the US sat on its butt saying they would not get involved in the European war. If Hitler had not declared war on the USA likely the USA may have lost the war.
Except for the invasion of Italy and the invasion of Normandy the only real war work the Americans were doing in Europe was working with Britain bombing German targets. The rest of the US forces did little in Europe beside bombing until they invaded Italy with British commonwealth forces and then Invaded Normandy. In the Invasion of Normandy US forces were only about 40% of the soldiers there as they only attacked 2 of the 5 beaches with the others being invaded by British commonwealth forces.
America really did its best in the Pacific Ocean fighting the Japanese, but again it was not only US forces there fighting the Japanese but credit where credit is due the Americans had the fire power and men and did the majority of the work. The Australians, New Zealanders, the Chinese as well as the British were there as well but in no where the numbers the US forces were, but American history seems to leave out the good work done by other countries. Where America really did shine was in military hardware manufacturing being a large industrialised nation away from most of the danger of the war able to manufacture without the threat of bombings turning out good quality products needed by the allies to keep up the fight.
The fact is America was important in their war efforts in Europe. You must also not forget the Russians took huge losses but likely did more than any country in destroying the German military. Britain and America did send a lot of military equipment to Russia at the start but Russia soon was producing huge quantities of equipment of their own. The fact is though the USA did not save Europe. It helped save Europe but Russian and British Commonwealth countries had kept Germany becoming strong enough where it could one day fight a major war on American soil. American war movies seem to inly show America invading Europe but the fact is they were the smaller force there on D Day. American bombing did a lot to destroy Germany's manufacturing but the British Commonwealth forces were doing the same. Britain set up a lot of the resistance fighting which did much to delay and confuse the Germans on D Day. British commandos destroyed the dry docks of St Naire in France so the Germans were not game to put big ships in the Atlantic as there would be no way of repairing them if they were badly damaged. It was the British who made things like the bouncing bomb that damaged the Manufacturing in the Ruhr Valley by bursting the dams and flooding the valley and the production of electricity. It was Britain that developed the earth quake bomb that could destroy underground manufacturing facilities that no US bomb could touch. It was Britain who was able to bottle up the very high quality Italian Navy in port too scared to come out after the British navy kicked its @ss. It was the British Maltese who kept the Italian and German air forces at bay for 3 weeks with 3 out of date biplanes until Britain could fly in better planes.
So hopefully now you realise how much the British Commonwealth forces were able to do before the USA even got involved in the war and quite possibly save the USA just as much as the US did to save them. The British forces were struggling but were hanging on and doing major damage to the German ambitions of world domination.