Which event marked the end of Japan's involvement in World War II?

The dropping of the atomic bombs on Hiroshima and Nagasaki in August 1945 is identified as the event that precipitated Japan's surrender and ultimately brought an end to its involvement in World War II. The immediate impact of these bombings led to devastating destruction and loss of life, and they played a crucial role in Japan's decision to capitulate. After these bombings, Japan faced insurmountable devastation and the potential for further bombings, which compelled the Japanese government to reconsider its military stance.

While significant battles such as the Battle of Iwo Jima were pivotal in the broader context of the Pacific War, they did not directly lead to Japan’s surrender. The signing of a peace treaty occurred later, post-surrender, and the assassination of key leaders did not influence Japan's formal withdrawal from the war. Thus, the atomic bombings directly catalyzed the end of hostilities, making this option the correct answer.