Hopefully this idea will make sense but as we all know it required a combination of the British, Russians and Americans to finally defeat the Axis Powers in WW2, and all three powers played important roles that affected how the others fought.
Eg - The British not suing for peace in mid 1940 before the Battle of Britain meant that Germany had to keep over 500,000 troops either in the west, or in Africa that could have otherwise been used to fight Russia, not to mention 2000 aircraft lost in the battle of Britain and the 6 week delay to Operation Barbarossa because the British helped out the Greeks and as a result the Germans had 6 weeks less time to fight before the Russian winter hit. The British also provided an important base for the American troop build up.
The Russian contribution is obvious as well as from what I have heard 70% of German soldiers that died in WW2 died on the Western Front and they kept Hitler very busy with both men and resources, so much so that Hitler could never really fortify the coast of France like he would have wanted, allowing gaps for the British and Americans to punch their way through.
The American help was very important as well, both with troop numbers but also materials. America helped push the Germans out of Africa and it was a combination of America and Britain that knocked Italy out of the war, something the British could not have done without American assistance.
So the general idea of this thread is that one of these nations does not get involved in the war, which combination of the other two nations would be most likely to win WW2?
Eg
Scenario 1 (British out of the war)
Britain sues for peace before Battle of Britain
Hitler invades Soviet Union in May 1941
Japanese attack USA in December 1941 and Germany declares war on America, with Britain not being in the war.
Scenario 2 (Soviets never in the war)
British keep fighting
Germany decides to try and knock Britain out of the war before even considering attacking the Soviets
America enters the war in December 1941
Scenario 3 (American never enters the war)
British keep fighting
Operation Barbarossa takes place in 22nd June 1941
Japan never attacks the USA, USA never enters the war but does keep supplying the British and to a lesser extent the Russians.
Which scenario is most likely going to see an allied victory, which one would most likely see a German victory?
Also those above scenarios are just guides, as there could be more plausible ways for one of the three nations not to enter the war that I did not think about.