Bombing Hiroshima and Nagasaki, Japan ended WW II.
Japan was preparing to defend their home islands against invasion by the US and allies in July, 1945. The leaders were preparing their people for suicidal resistance. Four experienced army divisions were withdrawn from Manchuria, China and 45 new divisions were built up in the first half of 1945. Many of these were immobile coastal defense units, but sixteen were mobile units. In all, there were 2.3 million Japanese Army troops prepared to defend the Japanese home islands, another 4 million Army and Navy employees, and a civilian militia of 28 million men and women.
Vice Admiral Onishi estimated Japan's casualties at 20 million deaths. The US Joint War Plans Committee estimated that "Olympic" would result in between 130,000 and 220,000 US-casualties of which U.S. dead would be the range from 25,000 to 46,000. This estimate was made with the recent results from the invasion of Okinawa. US Secretary of War Stimson put together a commission to make an independent estimate for him. The numbers were much worse: Allies would suffer between 1.7 and 4 million casualties in such a scenario, of whom between 400,000 and 800,000 would be dead. But better for the Japanese - casualties would have been around 5 to 10 million. Wikipedia on Invasion/defense of Japan
Compared to those expected casualty numbers the two atomic bomb attacks saved lives. Also the invasion would have lasted months to a year of misery for everyone in Japan and another year of war mobilization for the US - rationing, manufacturing airplanes instead of automobiles, etc.
President Truman made the right decision.
A book that sorts through the many revisionist histories of the Hiroshima/Nagasaki bombings: Hiroshima in History by Robert J Maddox. (No Amazon link, because the Big A is not responding to sign-in attempts.)
David French at National Review