My answer is ignoring the Pacific theatre.

No more than the statement "The Soviet Union was the reason the Allies won World War 2" can be made. The United States contributed a great deal to the material success of the Allies but the Soviets did the majority of the killing and the dying. Saying either one denigrates the contribution of the other.

A sort of a follow up question - is there an easily accessible breakdown of L&L over the years? I have read in several places that its impact on the Eastern front is extremely overstated, and that only after the Soviets have turned the tide in 43 did the majority of the material start to arrive, effectively only speeding up proceedings rather than being overly significant or decisive.

I wanted to add one more thought. The most important part of lend-lease to the the Soviets was probably trucks. This allowed Soviet industry to concentrate on weapons. As important as the idea of deep doctrine is without the trucks to haul the gas and diesel it doesn't work as well.

Germany would have lost the war all on its own due to both economic and strategic issues.

As most people know, one of the reasons why Germany was able to take France and Western Europe so quickly was due to the Blitzkrieg style of attack which was fast, surprised the enemy but also used a lot of Germany's resources via tanks and aircraft. After they took Western Europe and continued to expand, they had troops stationed in France, Norway, Holland, Belgium, North Africa (thanks Mussolini) and fighting on the Eastern Front (I'm sure I'm forgetting a few places). Now, Germany was a big country in 1939 but War requires massive manpower and Germany was stretched pretty thin. By the time they had invaded the Eastern Front, failed to take Russia and were holed up in places like Stalingrad, they were extremely short on men to actually fight.

Germany used conscription and recruited in many places they had invaded (France, Norway) and somewhat bolstered their army with this method. They also drafted some Poles into their army by declaring them ethnic Germans. By the end of the war, the age of conscription was 13 and the boys were used for the German Home Guard which failed to stop the Allied advance later in the war.

So, Germany had almost no one left to fight in their army by the end of the Stalingrad time in the war. Another problem that contributed to this was the lack of women working on the homefront. When Hitler came to power, he restored very traditional gender roles in German's society. A woman's place was in the home and therefore half of the potential working population was not utilized in Germany's factories. The problems of this are two fold. First, women working in factories would have allowed many strong healthy men to join the army, and second, it would have increased the economic output which Germany desperately needed by the end of the war. This is a stark contrast to the British and American economies who had many women working in factories or 'doing their part' to bolster the homefront.

So strategically, Germany was stretched too thin, had a hand in too many places, were running low on army recruits, resorting to using old men and young boys and had perfectly good men working in factories due to the adherence of traditional gender roles. However, economically, Germany was in much bigger trouble.

Britain and America waged a war against the German economy through the use of air raids in an attempt to reduce the German output (Strategic Bombing). They targeted major industrial centers and factories. Though many believe that these bombing contributed to the reduction of economic output, a study done by the British Bombing Survey Unit concluded that in fact these bombings had contributed very little. In fact, the German economy didn't need much help in destroying itself.

Hitler believed the war would be won efficiently and quickly which meant that the German economy was not fully mobilized (ie ready for war) until the war in the east stagnated. The system set up was rushed and worked poorly, and in fact Germany's greatest economic output occurred in the first few months of the war and declined steadily as the war continued.

By 1944, Germany's economy was collapsing all on its own. Germany didn't have very much access to physical resources and most importantly fuel. They had made a grab for oil when they invaded Russia but the attempt failed. Their production of aircraft, tanks, weapons and all other war resources were failing.

Germany would have lost the war all on its own. Though the first years of the war were categorized by the never-before seen Blitzkrieg style, after Germany opened the Eastern front, the war stagnated into the very familiar war of attrition which Germany could never have sustained against the much more powerful and more importantly, efficient British and American economies. It would be ignorant to claim that America contributed nothing to the war, but the Western Allied team was a very much united and equal front.

Further Reading:

Richard Overy, War and Economy in the Third Reich
Adam Tooze, The Wages of Destruction: The makers and breakers of the Nazi Economy
Alan Milward, German Economy at War

People generally are hesitant to say such things but i think World War2 was a very good example of a long war of attrition and so the available facts and numbers enable us to make such claims with sufficient certainty.

Without the US the war is very unlikely won. The direct and indirect impact of the US was too big. Without crunching the numbers some examples:

• The land lease help to UK and the Soviet Union was substantial and enabled both parties to stay in war. The help to the UK also helped the UK to help the Red Army better.

• Before the major contributition on the ground the US airforce diverted a lot of ressources from the Wehrmacht to the Defense of the Reich. Flak guns, personnel to man them and obviously airforce come to mind.

• The impact in Africa/Italy wouldnt be the same and could maybe be contained with a handful of german divisions. No way UK is able to mount a meaningful offensive against Italy mainland without the US. So the Wehrmacht doesn't need to bring so many troops to Italy.

• Every impact of the UK was enhanced by the US contribution. Every bit what happened on the Western Front helped the Red Army. Land, air and water. The amount of ressources used to fight and prepare for the western allies was substantial. Just the aircraft and fuel alone could have had major impact in the east, not even considering the thousands of guns and millions of personnel.

• The Uk alone is in no shape to launch any meaningful invasion of Europe and therefore doesn't require the same amount of defensiv meassures and occupation troops.

• Is the RAF even able to continue the air war over Germany alone?

• Just in 1944 July, 2000 tanks stay in the normandy including 100 Tigers and 800 Panther together with 400.000 man including at least 8 top notch divisions. Its even debatable if the Red Army is able to advance if these troops stay in belarus of ukraine during 44 Summer offensives.

From a pure numbers perspective there is no way the Red Army can sustain the damage inflicted by a Wehrmacht which doesn't need to divert ressources against the US war effort.

But there is a list for all three major countries to be made to show they were vital. Was the US neccessary? Certainly! Was the US effort more vital than the effort of the other allies? Likely not.