Actually, he was well aware of the damage caused by the Trinity Test, but there was a major war going on and millions of people were dying in Asia and the Pacific - the war had to end. He stated there was no difference between killing 100,000 people with thousands of small bombs and killing 100,000 people with a huge bomb - the U.S. Air Force previously flattened most of Japan's cities with strategic bombing through hundreds of B-29s loaded with thousands of incendiary bombs which killed hundreds of thousands of civilians in the process for months. He felt so little about the Japanese civilians dying by the hundreds of thousands from firebombing, including the March 9/10 1945 incendiary raid on Tokyo, which produced more civilian deaths and destruction to property than both nukes combined. It would be totally irrationally that he would spare the enemy cities from being nuked simply because it would cause huge civilian casualties, when nobody said the same for the 1,000 bomber raid all over that caused as much damage to civilian life and property. Most of the world public at the time approved the firebombing of Japan, so why not use the atomic bomb that do the same job with less risk to air force personnel?