The United States had one main concern and one lesser concern with regard to invading Japan at the end of World War II.
The lesser concern had to do with American public opinion and attitudes towards the war. By August of 1945, the war in Europe had been over for almost three months. Many in the US felt that the war ought to be over. Soldiers did not want to be taken from Europe and sent back into combat in the Pacific. The government wanted to avoid a long, drawn-out war that would test the patience of the public which was feeling that the end of the war was near.
The more important concern had to do with casualties. The Japanese had fought almost to the last man in battles such as those on Saipan, Iwo Jima, and Okinawa. In doing so, they inflicted many casualties on the Americans. American planners believed that an invasion of the Japanese home islands would be horrifically bloody. They estimated that as many as 1 million Americans would be killed or wounded. This, obviously, was something the US wanted to avoid if at all possible.