What significant milestone regarding nuclear weapons occurred during World War II?

During World War II, a significant milestone regarding nuclear weapons was the first use of nuclear weapons in warfare. This event marked a transformative moment in military history and global politics, as it demonstrated the destructive power of nuclear energy when harnessed for weaponry. The atomic bomb was dropped on Hiroshima on August 6, 1945, followed by a second bomb on Nagasaki on August 9, 1945. These bombings led to Japan's surrender and the end of the war, showcasing the immediate and devastating effects of nuclear warfare.

The other options represent important aspects of nuclear development and policy but do not pertain directly to the use of nuclear weapons during the war itself. The first nuclear reactor, created by Enrico Fermi in 1942, was part of the broader context of nuclear research, highlighting the beginning of the atomic age rather than its application in warfare. The Manhattan Project, which was crucial for the development of nuclear weapons, indeed began during the war, but it does not represent the actual use of those weapons. Lastly, nuclear arms reduction treaties emerged much later in history, focusing on the reduction and regulation of nuclear arsenals rather than their initial deployment.