My old history teacher - who had been in WW2 - summed it up well IMHO
The British brought TIME - from 1940 to late 1941 - no-one else was in the game - by start '42 two of the biggest players had joined us - after that there was only one logical ending
Russia brought SPACE & Manpower - they had a very rough ride to start with but by late '43 were grinding the Germans into the ground
The USA brought PRODUCTION & manpower - no-way the AXIS could win against such a wave of top quality kit - and they effectively took on the Japanese on their own at the same time as fighting the Germans
Any two would probably have won in the end (as in 14-18) - but all three together made it a certainty - even if it looked a bit different at the time in some places