First Atomic test detonation. Why it is was used?

Second world war was started on September 1st 1939 and lasted to September 2nd1945. A duration of six years and one day. This war costed 75 million deaths. It brought many things on the front line. One of those was the atomic Bomb. On October 9, 1941 US president D Roosevelt approved a program to develop Atomic bomb. And in 1942 J Robert Oppenheimer was invited to handle the task. He was appointed as director of this program. With the continuous and devoted enthusiasm in the short Period of three years on the expense of 2 billion dollars the first wave of atomic bombs was ready. Its first test detonation was carried out on July 16 1945 near Alamogordo new maxico. The heat of blast vaporized the stainless steel tower holding it. The sand was melted leaving an area of glass. scientists estimated that the force of the blast was equivalent to 70 million sticks of dynamite. By july 1945 the war in Europe was over. the Nazi german was defeated but in pacific the war against japan was raiding on. After the surprise attack on Pearl harbour america fought back their way to pacific. And decided to use this bomb against Japan. On August 6 1945 little boy, a 4 ton heavy enriched uranium gun-type fission weapon, was dropped on Heroshima. And three days later on August 9, fat boy, a plutonium implosion type weapon, was dropped on Nagasaki. It killed almost 2 hundred thousand people mostly civilians. Six days later On August 15 japan surrendered and on September 2 he signed the instrument of surrender effectively ending the war.