Why were other nation including the United States threatened by the Japanese attack on China

The Japanese invasion of China in 1937 marked the beginning of the Pacific front in what became the Second World War in 1939.

This invasion supposed the implementation of the expansionist ideology that had been developing in Japan since the victory in the Russo-Japanese War in 1905.

By defeating tsarist Russia in 1905, Japan began to acquire the condition of great power, which endorsed staying with a good part of the Asian colonies of Germany at the end of the First World War.

After his victory over the Russians, Japan added to its possessions the South Manchurian Railway and the strategic enclave of Port Arthur, which together with the island of Taiwan and the Penghu islands, occupied in 1895 in a previous conflict with China, would serve as a springboard for his undisguised ambitions on the continent.

Thus, in 1931, taking advantage of the "Mukden incident" (the blowing up of a section of railway track, organized by the Japanese army itself, which was blamed on Chinese saboteurs), Japan occupied all of Manchuria, separating it from the rest of China to create a puppet state, Manchukúo. The next step would be the attempt to dominate the vast Asian giants, ravaged by a civil war, undermined by corruption and victim of a centuries-old backwardness.

From the very beginning of the hostilities, the main supporter of China was the American president Franklin Delano Roosevelt, from whom it received an abundant support in the diplomatic and economic fields as well as in the military. In their eagerness to prevent the arrival of this massive and growing aid, the Japanese decided to initially capture the main ports of southern China and, in September 1940, Hai Phong, in Indochina.

This provoked a severe commercial embargo from the United States, which subsequently added the refusal to continue providing the vital oil both to keep the Japanese economy in place, and to be able to move its military machine.

The American reaction (which was shared by Western countries not aligned with Germany) was due to the fact that the Japanese imperialist attitude represented a threat to the international community, specifically for the nations of Asia and the Pacific, to break the peace that had been agreed after the First World War.