In the case of Japan, kill and be killed. The Japanese government was instructing civilians in tactics like strapping themselves with explosives and rolling under tanks. The Kamikaze pilots were just the tip of the iceberg -- lower-tech suicide attacks could call on old folks and children who wouldn't need much skill, training or a flyable airplane.
Your contention that the bomb was for Joe Stalin is absolutely wrong. We dropped the bomb to save the lives of our soldiers as well as the lives of the Japanese. Harry made the decision, no one else and it was a good humanitarian decision.
I agree that was the primary motivation, and that Truman made the right call. But sending a message to Stalin was also on the table. It was clear from the discussions at the summits that the Soviets and the Western allies were jockeying for position in the post-war world.
Hiroshima and Nagasaki were relatively undamaged from the previous waves of bombing -- they were chosen as targets rather than, say, Tokyo, because it made a more effective demonstration of the power of the Bomb. The Japanese leadership was the primary, but not the only, audience. What Truman did not know was how thoroughly compromised the Manhattan Project was, and that the Soviets would have the bomb themselves in only a few years.
I once knew a woman whose family was moved from Iwo Jima to Okinawa. Her brother was drafted and never heard from again. Her 7 year old sister killed herself and a group of Marines during the invasion of Okinawa with a grenade given to her by Japanese troops.
At that time, the only thing to do was kill them all and let God sort them out.
If you want to get a real insight into the mind of the Japanese war machine during WWII and increase your understanding of what we were up against read
"The Rape of Nanking"
It seems impossible that humans could be so cruel, but it happened just as the book describes.
It is a really good argument for the 2nd amendment.