The major event that led to the unconditional surrender of Japan, ending World War II, was the dropping of atomic bombs on Hiroshima and Nagasaki. This event had a profound impact on the course of the war and ultimately forced Japan to surrender.

Background:
- By the summer of 1945, Japan was facing significant military setbacks and was on the brink of defeat.
- The Allied forces, led by the United States, had already successfully invaded several Japanese-held territories and were advancing closer to the Japanese mainland.
- However, Japan's leadership, particularly Emperor Hirohito and the military high command, remained reluctant to surrender, despite the increasingly dire situation.

The Dropping of Atomic Bombs:
- In an effort to bring a swift end to the war and avoid a costly invasion of Japan, the United States made the decision to use atomic bombs.
- On August 6, 1945, the first atomic bomb, codenamed "Little Boy," was dropped on the city of Hiroshima. The bomb instantly killed an estimated 70,000 people and caused extensive destruction.
- Despite the devastation, Japan's leadership still did not immediately surrender.
- On August 9, a second atomic bomb, codenamed "Fat Man," was dropped on the city of Nagasaki, resulting in the deaths of approximately 40,000 people.
- The bombings demonstrated the devastating power of atomic weapons and left Japan with little choice but to reconsider its position.

The Impact and Surrender:
- The atomic bombings shocked the Japanese leadership and the population, who had not anticipated such destructive force.
- Emperor Hirohito, who had previously been reluctant to surrender, finally intervened and pushed for an end to the war.
- On August 15, 1945, Emperor Hirohito announced Japan's unconditional surrender in a radio broadcast.
- The official surrender ceremony took place on September 2, 1945, aboard the USS Missouri in Tokyo Bay, with General Douglas MacArthur accepting Japan's surrender on behalf of the Allied powers.

Significance:
- The dropping of atomic bombs on Hiroshima and Nagasaki was a significant turning point in World War II.
- It demonstrated the immense destructive power of nuclear weapons and marked the beginning of the nuclear age.
- The bombings also hastened the end of the war, saving countless lives that would have been lost in a prolonged invasion of Japan.
- However, the bombings remain controversial to this day, with debates over their necessity and the ethical implications of using such weapons.

Overall, the dropping of atomic bombs on Hiroshima and Nagasaki played a crucial role in compelling Japan to surrender and marking the end of World War II.