Attacking civilians is illegal even at the time of wars. Thus, the drop of atomic bombs on Japan, and other bombardments on Japanese cities including Tokyo by the United States must be illegal. This is not a which-is-the-justice problem, and the justice of the laws must be applied justly regardless of the final results of wars.
Peace was gone long before all those innocent people were murdered, and it didn't come back just because the Allies were declared winner. That was how we kept from spending more money and saving the lives of our own troops which i'm not totally opposed to. But chalking something like that up to a winning strategy with just a bunch of casualties of war is a grotesque oversimplification of a rightfully controversial event in history.
Base on the history, Soviet Union was seeking to enter war with Japan in hoping to share one of Japan's territory. Truman and USA congress agreed that they should adopt limited war to minimize casualties and to end war fast this is to prevent further intervention of Soviet Union which would have caused more casualties in a sense that Soviet Union will fight in the war with Japan to gain territories . In order to minimize the casualties, dropping of atomic bomb is the best choices for USA to end World War 2.
Japan was one of the most notorious countries in world war II that also killed thousands of Americans in pearl harbor. That triggered US to retaliate. Dropping atomic bombs forced Japan to surrender and stop colonizing more countries like Indonesia and China. War is always inhumane! If US didn't act quickly, tens of thousands of innocent lives of the countries that were colonized by Japan would die too!
In 1945, the United States dropped two atomic bombs on the cities of Hiroshima and Nagasaki. These bombs instantly killed tens of thousands of people and the negative legacy of these attacks have been felt throughout history since. Although this action pushed the modern world into a nuclear age, the unconditional surrender of Japan in August 1945 ended a war that some historians believe could have lasted 6-24 months longer if the USA decided to invade the island nation-state. This is why the dropping of the atomic bombs on Japan was a necessary action to secure world peace.