The correct answer is the bombing of Hiroshima and Nagasaki.
Towards the end of World War II, the United States decided to use two atomic bombs in order to bring a quick end to World War II. These atomic bombs were dropped on the cities of Hiroshima and Nagasaki, resulting in the unconditional surrender of the Japanese Empire. This brought in the atomic age, as it was clear America (along with other countries) had the capabilities to use powerful atomic weapons like the ones used on Japan. This would change the nature of warfare forever.