The Open Door Policy is the United States diplomatic policy established in the late 19th and early 20th century that called for a system of equal trade and investment and to guarantee the territorial integrity of Qing China. The policy was created in U.S. Secretary of State John Hay's Open Door Note, dated September 6, 1899, and circulated to the major European powers. In order to prevent the "carving of China like a melon", as they were doing in Africa, the Note asked the powers to keep China open to trade with all countries on an equal basis and called upon all powers, within their spheres of influence to refrain from interfering with any treaty port or any vested interest, to permit Chinese authorities to collect tariffs on an equal basis, and to show no favors to their own nationals in the matter of harbor dues or railroad charges. The policy was accepted only grudgingly, if at all, by the major powers, and it had no legal standing or enforcement mechanism. In July 1900, as the powers contemplated intervention to put down the violently anti-foreign Boxer uprising, Hay circulated a Second Open Door Note affirming the principles. Over the next decades, American policy-makers and national figures continued to refer to the Open Door Policy as a basic doctrine, and Chinese diplomats appealed to it as they sought American support, but critics pointed out that the policy had little practical effect.

History of United States foreign policy is a brief overview of major trends regarding the foreign policy of the United States from the American Revolution to the present. The major themes are becoming an "Empire of Liberty", promoting democracy, expanding across the continent, supporting liberal internationalism, contesting World Wars and the Cold War, fighting international terrorism, developing the Third World, and building a strong world economy with low tariffs.