Pros: Higher chance of Japanese leaders agreeing to a cease fire. Higher morale for the United States at home and on the frontlines. Ending the war earlier means less U.S. casualties. Also establishes a zone of control for the U.S. in southeast Asia that we might not have had with a regular truce. Destabilizes Japanese government that Americans presumed would have started the war in the first place. Showed the danger of nuclear weapons.
Cons: Purposefully killed civilians, either in the initial blast or in the nuclear fall out. Set nuclear weapons center stage for future conflicts between powerful nations. Showed the effectiveness of nuclear weapons.