Okay, so instead of losing most of his army in the Soviet Union, Hitler loses all of it. The USSR won the war in Europe, the US and UK only invaded to stop them from moving past Germany, but Germany was already defeated well before D-day happened. Let's assume the British decide to help Germany in Russia, good thing Russia has all that land, plenty of room for Britain's powerful navy... Oh wait, navies need water... well "Britain rules the waves" so at least they can stop overseas trade from getting to Russia... wait... Japan controls the Pacific with their huge Navy, and the only ones with a Navy in the Pacific big enough to take them on is the US which is now allies with Japan and enemies with Britain. Britain cant move ships to the Pacific without compromising the security of Britain itself (the US could cross the Atlantic and invade at any time) The USSR wins the war, like it did before, but they can't take Britain itself before the war becomes too costly to continue, Britian loses every last bit of their overseas territories. Most of Europe is "liberated" by the Soviets, who then install puppet regimes that report back to the Kremlin. China (formerly a major player on the Allies side) is fully conquered and horribly brutalized by Japan and the Soviets (who moved to help their allies once Germany was on the retreat) The US... doesn't really enter the war, I'm assuming they're giving supplies to Japan and the Soviets to stick to the spirit of the question, but in my honest opinion, had Britain allied itself with Germany, the US would have followed suit.

Conclusion. Allies win, the Soviet Union becomes so much more powerful with the added territories that it wins the cold war. The US, having benefited way less from WW2 than we did irl just can not compete.