Why did the United States use atomic weapons precisely at the beginning of August 1945?

Because at that time the United States was waging a fierce war with Japan, which could drag on for several more years. Thus, the United States wanted to force Japan to cease hostilities and agree to sign a peace treaty. One way or another, the explosion of atomic charges in Hiroshima and Nagasaki was a big disaster for Japan. Despite the fact that she left the war, in our time, some contemporaries consider such an act of the United States as a crime against humanity, since these explosions caused the death of hundreds of thousands of civilians.