Depends on what statistic and theater you use. The US played the Biggest part in the Pacific Theater, while USSR played the Biggest part in the European Theater, while Italy just sucked at fighting and made the Germans over stretch themselves. Then you have UK annd China who fought to hold on to their countries till the US and USSR got heavily involved. USA supplied all of its allies with weapons and equipment. Like I said it depends on what you are talking about when it comes to biggest part.