In the early years of World War II the United States had largely stayed neutral. There were examples of American pilots volunteering for foreign nations, such as the Eagle Squadron in Britain and the Flying Tigers in China. The US had stressed its neutrality in foreign ventures with the Quarantine Speech by FDR. As the Allies struggled under the weight of Axis operations, the US passed the Lend-Lease Act that provided war materiel and goods to Free France, United Kingdom, Republic of China, and later the USSR. In July 1941 US troops took over occupation in Iceland from the UK under the US-Icelandic Defense Agreement. Supporting the Republic of China, and hoping to stem Japanese expansion, the US placed economic sanctions on Japan, cutting off the precious resources the island nation needed.
The economic sanctions outraged the Japanese, helping instigate their plan for attacking the US Naval base at Pearl Harbor. The surprise attack on Pearl Harbor on 7 December 1941 changed the face of World War II. The attack led to the United States in joining the Allies. Popular history has attributed a quote to Japanese Adm. Isoroku Yamamoto regarding his fear of “awakening a sleeping giant,” and while there is no evidence that Yamamoto made this statement, it may have very well been his feelings regarding the attack.
The Japanese attack had truly awoken a sleeping giant. When World War II started in 1939 the US Army had 180,000 soldiers in its ranks. By the end of the war the US Army (including the attached Air Force) had 8.6 million soldiers. The American war-industry pumped out thousands of tanks, planes, and ships to fight the war. While in the early phase of the American involvement in the war the US suffered setbacks. The difference between the US and its adversaries when facing setbacks was the US could quickly replace losses. During the height of the war, the ship building sites were producing Liberty ships on an average of one every 42 days. In one unbelievable display of production the SS Robert E. Peary was launched four days and 15 hours after the keel was laid.
Like the US Army, the US Navy had seen a remarkable increase in size during the war. In December 1941 the navy had 350 major combatant ships. When the war ended the navy had 1,200 major combatant ships. In comparison, during World War II Japan had: 12 battleships, 15 fleet carriers, and 10 light/escort carriers. The US Navy had by V-J Day in August 1945: 23 battleships, 28 fleet carriers, and 71 light/escort carriers.
While it is debatable about the overall influence of the United States in the European Theater (by December 1941 the Germans had overextended themselves in the Soviet Union and it was a matter of time before the Soviets overwhelmed Germany), in the Pacific the answer was clear, the US had triumphed.