In 1945, during the Second World War, the United States dropped two atomic bombs on the Japanese cities of Hiroshima and Nagasaki. The matter has been subject to much debate and controversy. Indeed, it was the first time the atomic bomb had been used in warfare, resulted in a loss of around 120,000 lives, and left a legacy of ruin and disease from radiation. However, the US decision to use the A-bomb on Japan is justified because it was a way to prevent the loss of more lives and to end the war quickly.
Immediately after Japan sneak attacked the US naval base in Pearl Harbor on December 7, 1941, the United States declared war on Japan. The fight against Japan was a bloody and dangerous one - by Island Hopping, the US pushed the Japanese back, island by island. If the United States had not used the A-bomb and continued fighting Japan by method of Island Hopping, eventually the two sides would need to face off on the Japanese mainland.
Military advisers said that if the US invaded mainland Japan, it would cost between half a million to one million soldiers' lives, and the war would last until 1946. For the US, the use of the atomic bomb was economically justified because continuing the war would mean a continued drain on US resources and the loss of many lives.
The US also wanted to avoid fighting the Japanese because they were not ones known to give up easily and surrender. The Japanese had fought to the last man in Iwo Jima. Civilians would jump off cliffs in Okinawa rather than surrender. The two sudden blows to Japan by the A-bomb was enough to result in Japan's subsequent surrender and the end of the Second World War. Even after the first bomb was dropped on Hiroshima and Emperor Hirohito told the Japanese army to surrender, some military leaders attempted to overthrow Hirohito and continue the war.
In addition, putting a quick end to the war would prevent the USSR from taking over parts of the Pacific. One of the agreements at the Yalta Conference in 1945 was that the Soviet Union would be entitled to parts of Manchuria for joining the war against Japan. The a-bomb leading to the Japanese surrender would ensure that the USSR would have little or nothing to do with defeating Japan.
Though the use of the atomic bomb is arguably one of the most horrific events in modern history, devastating the two Japanese cities, decimating thousands and affecting thousands more, the US action to use the bomb is justified. It prevented the loss of more lives on both sides - the US and Japan - and was completely in the best interests of the United States.