Final answer:
During World War II, the significant events that occurred in Hiroshima and Nagasaki were the atomic bombings by the United States. The Hiroshima bombing on August 6, 1945, and the Nagasaki bombing on August 9 led to massive casualties and prompted Japan's surrender, thereby ending the war.
Step-by-step explanation:
The most significant event that occurred in Hiroshima and Nagasaki during World War II was the dropping of atomic bombs by the United States. These bombings are historic for their unprecedented destruction and the introduction of nuclear warfare. On August 6, 1945, a nuclear bomb nicknamed 'Little Boy' was detonated over Hiroshima, leading to the death of approximately 140,000 people. Additionally, large swathes of the city were completely flattened. In a further attempt to compel the Japanese to surrender, on August 9, a second atomic bomb known as 'Fat Man' was unleashed upon Nagasaki, claiming the lives of nearly 70,000 individuals immediately.
These events marked the first and only use of nuclear weapons in warfare and resulted in dire immediate and long-term humanitarian and environmental consequences. The attacks prompted Japan to consider surrender, with Emperor Hirohito realizing the nation could not withstand further bombings of such magnitude. This realization eventually led to Japan's unconditional surrender and the end of World War II, making these bombings a pivotal moment in modern history.