[Ryzorian]

The US calculated 1.1 million American dead and 4 to 6 million Japanese dead for the invasion of the Japanese main islands, set to begin in 1946. That was a conservative estimate based on casualties from Okinawa. Truman wanted to try to avoid that, and yet still force Japan to surrender, that was one of the decideing factors to drop the bombs.

They required undamaged cities to show the destructive power of these weapons, the US also wanted to demonstrate the willingness to utilize such weapons to win, if that's what it took to convince Japan about it's hopless situation. ( That's why the second blast).

In the end it was a numbers game, 300,000 Japanese dead verse 1,100,000 American and 4,000,000 to 6,000,000 Japanese dead. It's an ugly choice, wich would you make?