The Nine-Power Treaty (九カ国条約 Kyūkakoku Jōyaku?) or Nine Power Agreement (Chinese: 九國公約) was a treaty affirming the sovereignty and territorial integrity of China as per the Open Door Policy, after the Suzerainty system fell apart by the Western invasion in East Asia that outlawed Chinese capability for the "Close Door Policy" since Qing Dynasty, signed by all of the attendees of United of States of America, Belgium, the British Empire, China, France, Italy, Japan, the Netherlands, and Portugal to the Washington Naval Conference on 6 February 1922. The Nine-Power Treaty, concurrent with the Shangtung Treaty of the Washington Naval Conference, effectively prompted Japan to return territorial control of the Shandong Province (see Shandong Problem) to the Republic of China. The Nine Power Treaty Conference had a role to check upon Japanese aggression on the Battle of Shanghai,  but different to 1921, that the World peace in 1937 was disrupted by war in Europe and the non-interventionism by the United States, that China's hope for international intervention to Japanese invasion resulted no avail.
United States Secretary of State John Hay had issued the "Open Door Notes" of September–November 1899, followed by a diplomatic circular in July 1900, asking that all of the major world powers with vested interests in China declare formally that they would maintain an 'open door' to allow all nations equal rights and equal access to the treaty ports within their spheres of influence in China. Fearing that the European powers and Japan were preparing to carve China up into colonies, Hay also added provisions that Chinese territorial and administrative integrity should be maintained.
Although no nation specifically affirmed Hay’s proposal, Hay announced that each of the powers had granted consent in principle and treaties made after 1900 make reference to the Open Door Policy. Nonetheless, competition between the various powers for special concessions within China for railroad rights, mining rights, loans, foreign trade ports, etc. continued unabated.
The United States was especially leery of Japanese designs on China after the Russo-Japanese War (1904–1905) and the Twenty-One Demands (1915) and repeatedly signed agreements with the Japanese government pledging to maintain a policy of equality in Manchuria and China. These agreements concluded with Lansing-Ishii Agreement in 1917, which was soon shown to be completely ineffective.
During the Washington Naval Conference of 1921–1922, the United States government again raised the Open Door Policy as an international issue, and had all of the attendees (United States, Japan, China, France, Great Britain, Italy, Belgium, Netherlands, and Portugal) sign a new treaty which intended to make the Open Door Policy international law.
However, the Nine-Power Treaty lacked any enforcement regulations, and when violated by Japan during its invasion of Manchuria in 1931 and creation of Manchukuo, the United States could do little more than issue protests and impose economic sanctions. In November 1937, the signatories of the Nine-Power Treaty convened in Brussels for the Nine Power Treaty Conference after the outbreak of the Second Sino-Japanese War, but to no avail.
World War II effectively violated the Nine-Power Treaty.
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