You should maybe clarify your question. Every form of intervention or only in form of direct military intervention?

Without any intervention of the US the allies could not possibly sustain the war against Nazi Germany. The direct contributions to the Soviet and British war effort were just to big.

Let me explain why:

In the early stages of the war the primary contributions were Land-Lease, later they obviously joined the war directly by attacking German U-Boots boming German Mainland. Then the US started engaging German armies on the ground aswell. First in Africa then Italy and finally in northern and southern France.

The amount of ressources which went into dealing with all the results of the US intervention were huge. This is often overlooked by people who argue that the Red Army could defeat the Wehrmacht on its own.

I pretty certain this was impossible. Just some short overview over the implications created by the US entrance into the war.

Since this was a war of attrition after 1941 a simple look on the numbers shows that the Red Army is not able to outsustain the Wehrmacht. This means they are not able to replace the casualties and equipment losses they suffer to withstand the german attacks in the long run.

16.000.000 million tonnes of good were delivered to the soviet union. Including

• 7000 tanks

• 11.000 aircraft

• 400.000 trucks ( Germany produced 350.000 trucks during the war )

• Food

In the first years of war the Red Army was on the brink of collapse more than once. They lost roughly 60.000 tanks in the first 3 years of war and produced about 60.000 in the same time. Without land lease tanks this alone already would have created an issue. This is just an example to illustrate, that the land lease programm helped keep the red army functioning. Is it questionable if the Red Army would have been able to conduct their offensives without US trucks for example.

Besides these direct contribuations there were the indirect consequences for the Wehrmacht. The USA as ally of Great Britain now posed a big enough threat to western europe and africa to justify expensive defensive measures. Just as one example the Atlantikwall used huge amount of ressources.

Troops beeing deployed in the west:

• A third of the Luftwaffe was deployed against the allied bombing campagain

• 25% of the troops were deployed in the west in the early years up to 60% later

• 50% of the aircraft lost

Other consequences:

• Allied bombing reduced the german production

• Production shifts towards equipment which was specifically needed against bombers. Like search lights for example. Creating aircraft to deal with bombers.

• Diverting needed oil supplies or loss of them

A big chunk of these ressources would have straight went to the eastern front. Despite the decisive victory of the Red Army the numbers were closer than people might expect. The huge losses in men and equipment meant that the Red Army despite maximal effort in war production and conscription was only able to slighlty increase their numbers per year. The Wehrmacht on the other hand only managed to stay at the same numbers until 44. About 3 million men and 4.000 tanks per avarage. This increase in numbers of the Red Army eventually won the eastern front. But without US intervention the Soviet numbers decrease and the Wehrmacht numbers increase, thats not even counting the higher casualties the Red Army would sustain if the German army would field more ressources. Also not factoring in the decreased combat effictivness without Land-Lease supplies.

These are just some examples of how the US strengthend the Red army and weakend the Wehrmacht. I guess this shows the war was not sustainable for the Soviets.

I only focused on the Red army since your questions specifically asked if the Soviet Union would have won without the US. The position of Great Britain would have obviously bin weakened as well.