While the United States joined the war in 1941, two years after Europe exploded into conflict in 1939, the path to the Japanese bombing of Pearl Harbor, the surprise attack that threw the United States headlong into war, began much earlier. For the Empire of Japan, the war had begun a decade before Pearl Harbor.

On September 18, 1931, a small explosion tore up railroad tracks controlled by the Japanese-owned South Manchuria Railway near the city of Shenyang (Mukden) in the Chinese province of Manchuria. The railway company condemned the bombing as the work of anti-Japanese Chinese dissidents. Evidence, though, suggests that the initial explosion was neither an act of Chinese anti-Japanese sentiment nor an accident, but an elaborate ruse planned by the Japanese to provide a basis for invasion. In response, the privately operated Japanese Guandong (Kwangtung) army began shelling the Shenyang garrison the next day, and the garrison fell before nightfall. Hungry for Chinese territory and witnessing the weakness and disorganization of Chinese forces, but under the pretense of protecting Japanese citizens and investments, the Japanese Imperial Army ordered a full-scale invasion of Manchuria. The invasion was swift. Without a centralized Chinese army, the Japanese quickly defeated isolated Chinese warlords and by the end of February 1932, all of Manchuria was firmly under Japanese control. Japan established the nation of Manchukuo out of the former province of Manchuria.

This seemingly small skirmish—known—known by the Chinese as the September 18 Incident and the Japanese as the Manchurian Incident—sparked a war that would last thirteen years and claim the lives of over 35 million people. Comprehending Japanese motivations for attacking China, and the grueling stalemate of the ensuring war, are crucial for understanding Japan’s seemingly unprovoked attack on Pearl Harbor, Hawaii on December 7, 1941, and, therefore, for understanding the involvement of the United States in World War II as well.

Despite their rapid advance into Manchuria, the Japanese put off the invasion of China for nearly three years. Japan occupied a precarious domestic and international position after the September 18 Incident. At home, Japan was riven by political factionalism due to its stagnating economy. Leaders were torn as to whether to address modernization and lack of natural resources through unilateral expansion—the conquest of resource-rich areas such as Manchuria to export raw materials to domestic Japanese industrial bases such as Hiroshima and Nagasaki—or international cooperation—particularly a philosophy of pan-Asianism in an anti-Western coalition would push the colonial powers out of Asia. Ultimately, after a series of political crises and assassinations enflamed tensions, pro-war elements within the Japanese military triumphed over the more moderate civilian government. Japan committed itself to aggressive military expansion.

Chinese leaders Chiang Kai-shek and Zhang Xueliang appealed to the League of Nations for assistance against Japan. The United States supported the Chinese protest, proclaiming the Stimson Doctrine in January 1932, which refused to recognize any state established as a result of Japanese aggression. Meanwhile, the League of Nations sent Englishman Victor Bulwer-Lytton to investigate the September 18 Incident. After a six-month investigation, Bulwer-Lytton found the Japanese guilty of inciting the September 18 incident and demanded the return of Manchuria to China. The Japanese withdrew from the League of Nations in March 1933.

Japan isolated itself from the world. Its diplomatic isolation empowered radical military leaders who could point to Japanese military success in Manchuria and compare it to the diplomatic failures of the civilian government. The military took over Japanese policy. And in the military’s eyes, the conquest of China would not only provide for Japan’s industrial needs, it would secure Japanese supremacy in East Asia.

The Japanese launched a full-scale invasion of China. It assaulted the Marco Polo Bridge on August 7, 1937 and routed the forces of the Chinese National Revolutionary Army led by Chiang Kai-shek. The broken Chinese army gave up Beiping (Beijing) to the Japanese on August 8, Shanghai on November 26, and the capital, Nanjing (Nanking), on December 13. Between 250,000 and 300,000 people were killed, and tens of thousands of women were raped, when the Japanese besieged and then sacked Nanjing. The Western press labeled it the Rape of Nanjing. To halt the invading enemy, Chiang Kai-shek adopted a scorched-earth strategy of “trading space for time.” His Nationalist government retreated inland, burning villages and destroying dams, and established a new capital at the Yangtze River port of Chongqing (Chungking). Although the Nationalist’s scorched-earth policy hurt the Japanese military effort, it alienated scores of dislocated Chinese civilians and became a potent propaganda tool of the emerging Chinese Communist Party (CCP).

Americans read about the brutal fighting in China, but the United States not only the military capacity but the will to oppose the Japanese invasion. After the gut-wrenching carnage of World War I, many Americans retreated toward a policy known as isolationism and opposed any involvement in the massive conflagrations burning in Europe and Asia. But even if Americans had wished to intervene, their military was lacking. The Japanese army was a technologically advanced force consisting of 4,100,000 million men and 900,000 Chinese collaborators—and that was in China alone. The Japanese military was armed with modern rifles, artillery, armor, and aircraft. By 1940, the Japanese navy was the third-largest and among the most technologically advanced in the world.

Still, Chinese Nationalists lobbied Washington for aid. Chiang Kai-shek’s wife, Soong May-ling—known to the American public as Madame Chiang—led the effort. Born into a wealthy Japanese merchant family in 1898, Madame Chiang spent much of her childhood in the United States and had graduated from Wellesley College 1917 with a major in English literature. In contrast to her gruff husband, Madame Chiang was charming and able to use her knowledge of American culture and values to garner support for her husband and his government. But while the United States denounced Japanese aggression, it took no action.

As Chinese Nationalists fought for survival, the Communist Party was busy collecting people and supplies in the Northwestern Shaanxi Province. China had been at war with itself when the Japanese came. Nationalists battled a stubborn communist insurgency. In 1935 the Nationalists threw the communists out of the fertile Chinese coast, but an ambitious young commander named Mao Zedong recognized the power of the Chinese peasant population. In Shaanxi, Mao recruited from the local peasantry, building his force from a meager 7,000 survivors at the end of the Long March in 1935 to a robust 1.2 million members by the end of the war.

Although Japan had conquered much of the country, the Nationalists regrouped and the Communists rearmed. An uneasy truce paused the country’s civil war and refocused efforts on the invaders. The Chinese could not dislodge the Japanese, but they could stall their advance. The war mired in stalemate.