Japan was defeated and suing for peace, the atomic bombs were unnecessary. However, Truman and company had their eyes on the post World War II order and the Soviet Union. From Scott Ritter at lewrockwell.com:
As the world reflects on the decision by the US to drop two atomic bombs on Japan at the end of World War II, the reality is that the US nuclear enterprise remains the greatest threat to world peace.
Seventy-five years ago this week, two American B-29 ‘Superfortress’ bombers departed Tinian Island, in the northernmost part of the Mariana Islands, some 1,500 miles south of Tokyo, armed with the world’s newest and most horrific weapon: the atomic bomb. On August 6, a B-29 nicknamed the ‘Enola Gay’ dropped a single bomb containing 64 kilograms of highly enriched uranium over the Japanese city of Hiroshima. The bomb, nicknamed ‘Little Boy,’ detonated with the force of 15 kilotons of TNT. At least 66,000 people were killed outright, with another 69,000 wounded, many of whom subsequently died of their injuries.
Two days later a second B-29, nicknamed the ‘Bockscar,’ dropped a bomb containing 6.4 kilograms of plutonium over the city of Nagasaki. This weapon, nicknamed ‘Fat Man,’ detonated with a force of 21 kilotons, killing some 39,000 Japanese outright and wounding another 25,000, most of whom, like those injured in Hiroshima, later died from their wounds.
American historians have struggled with the morality of dropping weapons that could destroy a city and its population in one mighty blast. Over the years, a consensus has been reached that justifies the horror of using the atomic bomb on the grounds that it helped shorten the war with Japan and, in doing so, saved hundreds of thousands of American lives that would have been lost in any invasion of the main Japanese islands, along with the lives of millions of Japanese, who would have died defending their homeland.