The war in the Pacific essentially began on September 18, 1931, when Japan invaded Manchuria, which was known for its natural resources. The Japanese thought that from Manchuria, they could go on to control all of northern China. After Japan had established dominance in China, it could expand elsewhere. The Great Depression, Japan's population explosion, and the need to find new resources and markets to continue as a first-rate power, were other causes of the invasion.
The Japanese struck at a time when most countries were more concerned with the depression than with an invasion in far-off China. The United States introduced a policy of non-recognition, declaring that it would not recognize Japan's conquest.
The League of Nations did nothing but condemn Japan formally. Therefore, many consider the invasion of Manchuria as the real start of the war because aggression was not suppressed.
Since 1937, Japan had been buying cotton, gasoline, scrap iron, and aircraft equipment from the United States. After the “undeclared war” between Japan and China began in 1937, most Americans sympathized
with the Chinese.
In 1938, this led the United States to place an embargo on exporting aircraft to Japan. The government also froze all Japanese assets in the United States. Relations between Japan and the United States became increasingly tense in the fall of 1941.