General Dwight D. Eisenhower advised the Secretary of War, Henry L. Stimson, in July of 1945 that the Japanese were already essentially defeated, and therefore use of the bombs was unnecessary.

The same opinion was expressed by Fleet Admiral William Leahy (the Chief of Staff to the President), General Carl Spaatz (commander of the U.S. Strategic Air Forces in the Pacific), and Brigadier General Carter Clarke (the military intelligence officer who prepared intercepted Japanese cables for U.S. officials); Major General Curtis LeMay ; and Admiral Ernest King, U.S. Chief of Naval Operations, and Fleet Admiral Chester W. Nimitz, Commander in Chief of the Pacific Fleet.

Eisenhower wrote in his memoir The White House Years, "in 1945 Secretary of War Stimson, visiting my headquarters in Germany, informed me that our government was preparing to drop an atomic bomb on Japan. ..I voiced to him my grave misgivings, first on the basis of my belief that Japan was already defeated and that dropping the bomb was completely unnecessary, and secondly because I thought that our country should avoid shocking world opinion by the use of a weapon whose employment, was I thought, no longer mandatory as a measure to save American lives"

The United States Strategic Bombing Survey, after interviewing hundreds of Japanese civilian and military leaders after Japan surrendered, reported:
"Based on a detailed investigation of all the facts, and supported by the testimony of the surviving Japanese leaders involved, it is the Survey's opinion that certainly prior to 31 December 1945, and in all probability prior to 1 November 1945, Japan would have surrendered even if the atomic bombs had not been dropped, even if Russia had not entered the war, and even if no invasion had been planned or contemplated."

This information is all sourced from various books in Wikipedia.