Neither, the first battle was called Battle of Bull Run
The use of atomic weapons.
"“That single bomb, weighing 8,900 pounds, wiped out nearly five square miles of Hiroshima–60 percent of the city. More than 78,000 of the city’s total population of 348,000 were killed; an estimated 51,000 were injured or missing.” " “The Japanese announced their acceptance of unconditional surrender on August 14. World War II officially ended at 10:30 a.m. Tokyo time, September 2, 1945, when Japanese emissaries signed the surrender document aboard the battleship USS Missouri in Tokyo Bay.” " And ". “Atomic bombs have been used only twice in war—both times by the United States against Japan at the end of World War II.”
Hope this helps
