Often the moments that change the course of human history aren't understood until the consequences of such actions are already realized. That wasn't so with the atomic bomb — President Harry Truman knew the invention was going to change the world forever when he authorized the attacks on Hiroshima and Nagasaki in World War II.

Truman recalled the moment he told Soviet Union leader Joseph Stalin about the plan to drop the bomb — a memory he scrawled on the back of a photograph rediscovered by presidential historian Michael Beschloss:

Harry Truman note on back of photograph about telling Stalin about US atomic bomb, July 1945: pic.twitter.com/GqLSQTcnHb — Michael Beschloss (@BeschlossDC) May 27, 2016

In which I tell Stalin we expect to drop the most powerful explosive ever made on the Japanese. He smiled and said he appreciated my telling him — but he did not know what I was talking about — the atomic bomb!

Around 140,000 people were killed or died within months of the August 6, 1945 attack. Three days later, 80,000 people were killed when a second atomic bomb was dropped on Nagasaki. On Friday, President Obama became the first sitting U.S. president to visit the site of the Hiroshima bombing since Truman's decision. Jeva Lange