The Allies declared after the Potsdam Summit that the only surrender they would accept from the Axis powers was an unconditional surrender, that is, one without any conditions attached it to be the surrendering party.
Truman was fully briefed about Japan's resolve and had to weigh that in deciding where and how to force Japan into an unconditional surrender. If they had not dropped the bomb, preparations were being made to land an invasion force.
The main reason the United States decided to build a canal across Panama instead of Nicaragua was that both the land and the building rights were far less expansive. hope this helps (:
