Sorry this is one of my pet peeves and shows typical arrogant eurocentric thinking that the only place in the world is europe.
The British Empire at the time covered a 1/4 of the globe. Some of which was in East Asia and at war with Japan, a major power given what they where doing in China, Singapore and Hawaii. Japan was a very credible threat and was on the verge of taking over the whole of the Western Pacific.
If the Japanese had attacked the Russians instead of the Americans and British, Russia would have had its back against the wall. No American aid, and a two way front. Very hard to relocate factories to Siberia if it was flying the Japanese flag.
Germany was fighting a multi-front war, it was up against the British Empire and the USA eventually (minus that fighting Japan). That was not a good position to be in given the British Navy, Radar and the British 'lucky' ability to read the codes faster then the enemy commanders got them (first by mathematicians later by actually having the machine, some of the first computers).
Russia on the other hand was against one foe on one front, with the backup of two major players keeping the Japanese from them.
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