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The Nine-Power Treaty (九カ国条約 Kyūkakoku Jōyaku?) or Nine Power Agreement (Chinese: 九國公約) was a 1922 treaty affirming the sovereignty and territorial integrity of China as per the Open Door Policy. This was after the Suzerainty system fell apart after the Western invasions of the Opium Wars, that outlawed the Chinese "Closed Door Policy" into China of the former Imperial Qing dynasty.
The Nine-Power Treaty was signed on 6 February 1922 by all of the attendees to the Washington Naval Conference: the United States, Belgium, the British Empire, Republic of China, France, Italy, Imperial Japan, the Netherlands, and Portugal.
Open Door Policy
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 Qing dynasty Imperial China for railroad rights, mining rights, loans, foreign trade ports, and privilege 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 the rest of Mainland 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, Republic of China, Imperial Japan, France, Great Britain, Italy, Belgium, Netherlands, and Portugal) sign the Nine-Power Treaty which intended to make the Open Door Policy international law.
The Nine-Power Treaty, concurrent with the Shangtung Treaty of the Washington Naval Conference, effectively prompted Japan to return territorial control of Shandong province, of the Shandong Problem, to the Republic of China. The Nine-Power Treaty was one of several treaties concluded at the Washington Naval Conference. Other major agreements included the Four-Power Treaty, the Five-Power Treaty, and the Shangtung Treaty.
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. It did have a role in checking Japanese aggression in the Battle of Shanghai. 
World peace in 1937 was disrupted by war in Europe and the non-interventionism of the United States, although the United States efforts and treaties did considerably slow down their advances. China's hope for international intervention to Japanese invasion was not met.
World War II effectively violated the Nine-Power Treaty.
- Baer, George (1996). One Hundred Years of Sea Power: The U. S. Navy, 1890-1990. Stanford University Press. ISBN 0-8047-2794-5.
- Lamb, Margaret (2001). From Versailles To Pearl Harbor: The Origins of the Second World War in Europe and Asia. Palgrave Macmillan. ISBN 0-333-73840-3.
- Myer, Carl L (1936). Treaty relations between the United States and the far east: (with special reference to the four-power, five-power, and nine-power treaties). Library of Congress Legislative Reference Service. ASIN B0008D24WG.