"vj-day" what it is, how had happen and what caused that

How it happened:
On August 6th, 1945, the United States military dropped an atomic bomb over the city of Hiroshima, Japan in an effort to force Japan into an immediate, unconditional surrender. Instead of immediately surrendering, the Japanese government debated what to do. So, the U.S. dropped a second atomic bomb on August 9, 1945 over the city of Nagasaki, Japan.

What caused that:
On August 15, 1945, news of the surrender was announced to the world. This sparked spontaneous celebrations over the final ending of World War II. On September 2, 1945, a formal surrender ceremony was held in Tokyo Bay aboard the USS Missouri. At the time, President Truman declared September 2 to be VJ Day.