Interesting that no one has mentioned (unless I missed something) the only power that might be considered “least acknowledged” for BOTH the Axis and Allied causes: Romania. As an Axis power, Romania provided the largest contribution of manpower after Germany in the war against the USSR, and I think the third largest (after Germany and Japan) army, period, for the Axis powers. When Romania defected, Hitler is suppose to have for the first time said “We’ve lost the war.” This was probably also due in no small part to the loss of strategic resources as well as the manpower, but in simple truth, the Germans could not have fought as long as they did without the Romanian blood and muscle.
Then, when the Romanians switched sides, they provided the 4th largest national army against the Germans – following the USSR, USA and British Commonwealth. (In terms of raw numbers, Poland is said to have provided the 4th largest amount of manpower in the war against Axis. But these were dispersed among the armed forces of many countries. Moreover, I suspect there may be some “double-dipping” going on: was a soldier who fought for Poland, escaped to France and joined the Polish exile forces there, then escaped to England and joined the forces there - was such a man counted once, or three times? I don’t know.)
Yet the name of Romania tends to draw only blank expressions in you ask the “average Joe” about WWII (outside of Romania and Hungary, of course).