In 1941, Japan attacked Pearl Harbor in Hawaii, a water base of the United States, and destroyed or damaged many ships and airplanes. This started the United States' involvement in World War II. American and Japanese forces fought each other in the Pacific. Once airbases were established within range of the Japanese mainland, America began to win, and started dropping bombs on Japanese cities. America was able to bomb most of the important cities and quickly brought Japan close to defeat. To make Japan surrender, the United States dropped two atomic bombs on the cities of Hiroshima and Nagasaki, killing 150,000 Japanese citizens. Soon after this the Soviet Union began to fight against Japan, and the Japanese army in Manchuria lost. Japan surrendered and gave up all the places it took from other countries, accepting the Potsdam Proclamation. The United States occupied Japan and forced it to write a new constitution, in which it promised to never go to war again.