After the German invasion of Poland in 1939, Britain and France, allies of Poland, declared war on Germany. Soon after, France was also invaded and fell to Nazi Germany in 1940. After the surrender of Paris, Germany launched a campaign to bomb Britain into submission that lasted from July 10, 1940 to October 31, 1940. Although America had yet to enter the war, they sent supplies across the Atlantic to help the British during the constant German bomb raids. After its failed conquest of Britain, Germany attempted to invade the USSR in 1941. When the invasion failed, it spelled doom for Germany although the war would end in 1945.
America entered the war after a devastating Japanese surprise attack on Pearl Harbor, the base that was the home to the US Pacific Fleet, on December 7th, 1941. However, the Japanese failed to eliminate several primary targets, including three Essex-class carriers. These carriers and the remainder of the Pacific Fleet held off the Imperial Japanese Navy as the American war machine churned out new destroyers, carriers, cruisers, and battleships.
The Battle of Midway was the turning point in the Pacific theatre. Before the Battle of Midway, an intercepted message showed that the Japanese were plotting to attack "Island AF". Some US military officials thought that "Island AF" was the Aleutian islands in Alaska. However, the real target was soon revealed when the US base at Midway sent out a message announcing that it was low on water. American code breakers intercepted another Japanese message that showed that "Island AF" was low on water. The small American task force sailed to Midway to intercept the Japanese attack force. They sank four Japanese aircraft carriers, the Hiryu, Akagi, Soryu, and Kaga at the cost of only one aircraft carrier, the USS Yorktown.
After the Battle of Midway, the United States went on the offensive, sending in marines to capture and hold Japanese island bases with the support of battleships from the sea. The United States military used an island hopping strategy to close the distance between the two countries. The marines conducted amphibious operations, going ashore Japanese islands and capturing them. After capturing islands near the Japanese home islands, the US military would launch a full scale attack on Japan. However, this method was costly and other strategies were being considered.