The United States boasted vast plains, lush river systems, and abundant mineral resources, all sufficient to make it prosperous and powerful. America was an independent continent, isolated from other nations. After conquering the Native Americans and defeating Mexico, the Americans rested on their laurels. Their foreign policy was militarily and economically detached, avoiding involvement in wars and supporting other nations without benefit.

    After the outbreak of World War II in 1939, the world was in turmoil, but the United States remained remarkably calm. In Europe, however, Britain and France were frantically borrowing money from the United States to fight the war, placing large orders for aircraft, tanks, and military supplies. However, the Americans were not "hardcore" allies. They believed in the principle of isolation and prioritized their own interests, thus opening their doors to the Axis powers.

    In the early stages of World War II, the United States was Japan's largest supplier of oil, rubber, and steel. In 1937, the United States exported nearly $300 million worth of supplies to Japan. 90% of Japan's oil and 80% of its military supplies came from the United States. Trade between the United States and Germany was also very frequent, with nearly $300 million in goods exported. In 1939, 20% of U.S. oil exports went to Germany. The United States also continuously sold trucks, aircraft engines, gasoline, and military supplies such as American-produced aluminum, rubber, ball bearings, and aircraft anti-explosion agents to Germany. At the beginning of the war, no one believed Hitler could achieve such a great victory. It was not until France surrendered and London was bombed that the United States began to worry. The United States had a keen eye on the global situation. The Axis powers were ferocious and aggressive, and the invaded countries were helping the United States resist them. If these countries collapsed, the United States would not be spared. Therefore, the United States continued to provide additional support to the invaded countries of China, Britain, and France.

    China was defeated by the Japanese invasion in 1937. Starting in 1938, the United States provided aid to China, and arms purchased by the Nationalist government were shipped to China from Hong Kong and Vietnam. In 1938, the United States began providing loans to China, with the first loan amounting to $25 million. This amount increased to $100 million the following year, plus $60 million for the purchase of metals such as tungsten and tin. In 1940, the United States provided China with 100 fighter jets specifically for protecting the Burma Road.

    The United States has a long history of lending money to other countries to fight wars. Britain and France borrowed heavily from the United States before the war, and the US repaid them in full. Britain's domestic production was severely impacted by the war with Germany, leaving its military underequipped. In 1940 alone, the US sent Britain 970,000 rifles, 90,000 machine guns, and thousands of various artillery pieces. Britain also placed an order for 15,000 aircraft, to be delivered within two years. These fighters later became the capital of the British Air Force in the air war against Germany. In 1941, the US enacted the Lend-Lease Act, directly loaning Britain $7 billion to finance the import of military supplies. It can be said that Britain's ability to hold out before the US entered the war was entirely due to the US's support. France also ordered tens of thousands of fighter planes from the US before the war, but due to France's rapid surrender, the order was subsequently unfulfilled.

    The US military was very small before 1939. In 1920, the U.S. National Defense Act authorized the creation of a 280,000-man army, the largest peacetime force in history. However, until 1939, Congress never appropriated funds for more than half that strength. Most funds available for new equipment went to the fledgling Air Force. For much of the interwar period, the U.S. military remained small, composed of resilient, long-serving volunteers scattered across small garrisons in the continental United States, Hawaii, the Philippines, and Panama.

    In 1940, France, the leading European power, surrendered after holding out for less than two months, a stark contrast to World War I. Following France's surrender, only Britain remained stagnant in Western Europe, while the entirety of Eastern and Southern Europe was dominated by German forces. Meanwhile, the Soviet Union and Germany had signed a non-aggression pact, ensuring peace between the two countries. American leaders understood that this war was unlike any other, and that the United States could not remain immune. At this time, the United States was still largely peaceful, with isolationism and pacifism predominating in Congress. Even President Roosevelt dared not publicly advocate for war participation. However, US high-ranking officials and military commanders knew full well that war was inevitable and the US could not remain immune.

    First, Japan had already achieved significant victories in Asia, putting the Chinese Nationalist government in imminent danger. Furthermore, Japan's ambitions extended beyond this. It had a strong interest in Southeast Asia, a region that was within the British, American, and French sphere of influence and a crucial oil-producing region that could not be lost. Second, Germany had nearly completed its unification in Europe, leaving Europe with two possible outcomes. First, Britain's surrender would spark German interest in Greenland and Canada in America, potentially encroaching on the American continent—something the US did not want to see.

    Second, a conflict between Germany and the Soviet Union. Intelligence at the time indicated that such a war was only a matter of time. If war broke out between Germany and the Soviet Union, Europe would be faced with a repeat of Germany's two-front war in World War I. The US needed to intervene to reap the benefits.

    Under these circumstances, although Americans sought isolation, the top brass had already begun a series of resource mobilizations. The US experienced an economic crisis in the 1930s, but slowly recovered under the adjustments of Roosevelt's New Deal. After the Great Depression of 1933, U.S. production began to rebound. By 1939, the year World War II broke out, the United States' industrial output accounted for over one-third of the world's total, with production capacity for automobiles, aircraft, and ships exceeding that of the entire world combined. Arms sales to other countries created numerous jobs in the United States, leaving factories teeming with orders and cultivating a large number of skilled workers and technicians.

    From the outbreak of World War II in 1939, the United States was preparing to expand its military. Army Command began sending officers to observe the European battlefields. Upon their return, they updated the U.S. military's training manuals, basing everything on actual combat in Europe. During the initial phase of military training, the U.S. military conducted large-scale, grassroots non-commissioned officer training. This two-year, intensive training cultivated a cadre of grassroots troops with a deep understanding of tactics and training. These talents would later become the backbone of the tens of millions of U.S. troops.

    As for the Navy and Air Force, these two technical branches competed on national strength. In terms of scientific research, industrial capacity, and personnel quality, the United States was unrivaled. Within two years of entering the war, the United States produced tens of thousands of aircraft and hundreds of large warships. With a large number of young students eager to join the military, the US Navy and Air Force quickly became the world's leading force. The US Navy and Air Force had become a stumbling block to the expansion of Japanese militarism, and a US-Japan war was inevitable.