Japan’s attack on Pearl Harbor on December 7, 1941, was a shocking display of its ruthless military tactics during World War II. Over 2,400 U.S. servicemen died during the surprise attack, thrusting the United States into a war that shook the world to its core. But it wasn’t Japan’s final attack on American soil. Roughly two years later, the Japanese came up with a plan to harness air currents to create the world’s first intercontinental weapons: balloon bombs.