Which explains why some scholars argue that truman's decision to drop atomic bombs on japan was unwarranted?

There was not much knowledge behind the atomic bomb, especially because it was top secret, therefore it was unwarranted on Truman's part because the Japanese could not have known how destructive the bombs would be. Many believe that one of the subliminal messages from the atomic bombs was to show the might of the U.S military to the Soviet Union. 

What research did you learn from?