Why is it that the USA and the USSR can choose to enter the war fairly early in the game? In reality both of them entered the war because they were attacked, not because they chose to fight against Germany.
The USA could perhaps have entered without Pearl Harbor, because by that point they were already sending a lot of material and the stuff to Britain and were involved a bit in the war in the Atlantic (the first US casualties in WWII were before Pearl Harbor, in the Atlantic). But the general population was still a bit isolationist and it could be argued that they wouldn’t have entered until a lot later without the surprise attack. If London did fall, I don’t see how that would prompt the USA to enter the war, because at that point they would have essentially been alone against Germany (collaboration with the USSR was difficult due to them being in opposite sides of Europe).
From a gameplay perspective, allowing the USA to declare war so early severely limits Japan’s options by forcing an early confrontation.
The USSR almost definitely would have stayed out of the war entirely without Barbarossa, so I don’t see why they can declare war so early (though this isn’t as much of an issue as the USA because the German player needs to attack Russia early). But having such an early limit gives the German player a timetable that isn’t realistic for them to have to follow.