Which event occurred on August 15, 1945?

August 15, 1945, marks a significant moment in history as it is the date when Japan officially announced its surrender to the United States, effectively bringing an end to World War II in the Pacific. This announcement followed the atomic bombings of Hiroshima and Nagasaki in early August, which played crucial roles in compelling Japan to capitulate. The surrender led to the official signing of surrender documents later, but the announcement on this date was pivotal in signaling the conclusion of the war.

The other options highlight events that either occurred in different contexts or times that do not relate to World War II or Japan's surrender. The Treaty of Versailles was signed after World War I, not II, and the assassination of Emperor Hirohito is a historical event that did not happen, as he was not assassinated and lived until 1989. Additionally, the end of World War I was marked by events and treaties issued in 1918 and 1919, completely unrelated to events from 1945. Therefore, the announcement of Japan's surrender is the definitive and historically significant event associated with August 15, 1945.