1. Atomic Bomb dropped on Hiroshima and Nagasaki- In August of 1945, the US decided to drop an atomic bomb on the city Hiroshima, and then Nagasaki. 100,000 people were killed in Hiroshima and thousands more are killed in the second bombing. I ranked this as my most important development because it caused the Japanese Government to accept the Potsdam Declaration, causing Japan to surrender the war. It also caused thousands of soldiers to return home from the United States after the war.