Not to advocate the triggering of mass annihilation in anger, but it looks like the bombs that exploded over Japan in WWII - and caused it to surrender - might have saved more than they destroyed. By Joseph Coates in a comment to Hiroshima: The lost photographs:
The use of atomic weapons for the first time on Earth by the U.S. against the Japanese Empire and its civilian cities has always been a frustrating horror for me. I am alive because of it.

My father was an 18 year old kid (on a "great adventure") and unaware of the potential fate that would await him as he sailed with thousands of other soldiers in late October on a troop ship steaming across the South Pacific to invade Japan in Operation Olympic for "X-Day", as it was called.

Instead of probably being wounded or more likely killed while landing on the heavily defended mountains and beaches of Ōsumi Province or Satsuma on the island of Kyūshū in a massive invasion that was to make D-Day look like a skirmish — he helped rebuild Japan.

Names like Miyazaki, Ariake, and Kushikino, the three main invasion points, would be in our history books. The beaches of D-Day would be remembered with the beaches of X-Day, named after car brands: Austin, Buick, Cadillac, Stutz, Winton, Zephyr.

The Japanese had prepared an all out last stand defense of Japan, Operation Ketsugo, with no reserves.

Had the invasion happened, it is estimated that millions of American soldiers would have been killed or wounded and tens of millions of Japanese soldiers and civilians. The Pentagon ordered 500,000 Purple Hearts in preparation for the invasion of Japan and usually the military does not order enough of anything. Until just a few years ago, these unused invasion of Japan Purple Hearts were the Purple Hearts all recipients of all subsequent U.S. wars were given.

My father and countless other allied soldiers, Marines, and sailors never invaded Japan. He was switched to the Corp of Engineers after the Japanese Empire surrendered and promoted to sergeant so he could coordinate construction work for the Tokyo airport. After this, he went to college under the GI bill, got married, and raised a family. He helped run a number of companies, including a company founded by his brother (who was in the Navy during the war) which invented and sold, and still sells, important technology used in the manufacturing of microchips and later, LCD displays. (Your computer, its screen, your cell phone, ipod, et al could not have been made without it.)

He is alive, my family is alive, I am alive because the U.S. did not invade Japan. Many more U.S. and Japanese citizens and their families are alive too. The technology, medicines, and other inventions since WWII developed during or later by my father’s generation have saved and enriched billions of lives.

But if the U.S. had invaded Japan, would we have lost some crucial figures of that generation and the economic, technological, and creative prosperity of the late 20th Century?