In regards to Japan, the United States can decisively be accredited to winning World War Two. Their "island hopping" campaigns in the South Pacific were nothing short of brilliant. Nevertheless, Japan would have been defeated with or without two nuclear bombs dropped on Nagasaki and Hiroshima in 1945.
And likewise, Britain and France acting together, could not have carried out Operation Overlord or the 1944 D-Day landings without the United States. America was vital to Europe's success due to its financial contributions by means of Lend-Lease. Much of Europe lay in ruins and was in effect, financially bankrupt. The United States contributed over 16 million troops during the war, only second to Russia. And from 1943 and onwards, most campaigns on the western front were all American led under Dwight D. Eisenhower.
The European Theatre of World War Two and its victory can be attributed to an all Allied effort, with the United States contributing no more or no less than other participating countries. In a military point of view, the Russians bore the brunt of Allied fighting and as a consequence, received the most casualties. The Red Army was extremely instrumental in Germany's defeat.
The Pacific Theatre of World War Two and its victory can be 100% attributed to the United States.
Lend-Lease (Public Law 77-11) was the name of the program under which the United States of America supplied the United Kingdom, the Soviet Union, China, France and other Allied nations with vast amounts of war material between 1941 and 1945 in return for, in the case of Britain, military bases in Newfoundland, Bermuda, and the British West Indies. It began in March 1941, over 18 months after the outbreak of the war in September 1939.
A total of $50.1 billion (equivalent to nearly $700 billion at 2007 prices) worth of supplies were shipped: $31.4 billion to Britain, $11.3 billion to the Soviet Union, $3.2 billion to France and $1.6 billion to China. Reverse Lend Lease comprised services (like rent on air bases) that went to the U.S. It totaled $7.8 billion, of which $6.8 billion came from the British and the Commonwealth. Apart from that, there were no repayments of supplies that arrived before the termination date, the terms of the agreement providing for their return or destruction. (Supplies after that date were sold to Britain at a discount, for £1,075 million, using long-term loans from the U.S.) Canada operated a similar program that sent $4.7 billion in supplies to Britain and Soviet Union.