Dropping the Atomic Bombs on Japan (1945)

President Truman's decision to drop atomic bombs on Hiroshima and Nagasaki killed an estimated 200,000 people — mostly civilians — and remains the only use of nuclear weapons in warfare. Defenders argue it ended World War II without a land invasion that could have cost millions more lives, a claim that outperforms #2 in terms of immediate casualty avoidance. Critics contend Japan was already on the verge of surrender and the bombings were unnecessary, constituting the largest act of mass civilian killing in a single week in history, with over 60% of those casualties occurring within the first three days.