The biggest act of Terrorism ever.
Considering how many lives would have been lost in the invasion of Japan it could be argued that it was an act of mercy. An invasion would have cost millions of Japanese lives. The behavior of both the Japanese military and citizens on Okinawa leaves little room for doubt of the genocide that would have occurred.
Some say a demonstration of the bomb's power would have been more humane, but the Japanese refusal to surrender after the first atomic bomb was dropped demonstrates this strategy would not have worked.
Also, the comment "greatest act of terrorism ever" sounds like a ridiculous cry for attention. There was a war between the two counties at the time. The bombing was a military operation. If you are going to include war time operations, why not the bombing of Dresdan or the Siege of Leningrad, both of which cost many more lives.