A VVSO I once met and who had fought his way from Normandy to Hamburg said when asked:
"The British bought time - if we'd surrendered in 1940 the US would never have come into Europe and the Russians would have been pushed way over the Urals. The Russians bought space and brought manpower - we'd never have won in Europe without the meat-grinder on the Eastern Front. The Americans brought technology and manpower - we'd still be sitting here staring across the Channel without them. Without all three fighting together the German would have probably won......."
Pretty fair summary I think