The result of these commercial, religious, and political connections was that relations between the U.S. and China were good for much of American history. In the late 1800s, the powers of Europe and Japan were expanding their colonial empires. Some of them wanted to break China up into colonies, but U.S. leaders believed it would be better for American interests if China remained independent and united. So, the U.S. supported an “Open Door” policy, which meant that China would have an “open door” to foreign investment and trade, but no nation would control it. This was a fundamental part of U.S. policy toward China through the end of World War II, and it kept China from fragmenting and limited foreign exploitation.
When Japan tried to expand its empire in the early 1930s, the U.S. believed this violated the “Open Door” policy. America’s opposition to Japanese expansion ultimately led the U.S. to deploy its Pacific Fleet to Pearl Harbor, where Japan attacked it on December 7, 1941. Even before then, American volunteers, such as the famed “Flying Tigers,” were fighting in China. When the U.S. entered the war, it flew squadrons of B-29s from China, and sent it substantial amounts of aid. After the war, it was the U.S. that insisted that China be included as one of the five Permanent Members of the U.N. Security Council.
Sino-American relations were not always good. The U.S. passed the Chinese Exclusion Act in 1882; this marked the first time the U.S. had restricted immigration. The U.S. later prohibited Chinese immigrants from obtaining citizenship because of their race, which it had never done before. When U.S. forces joined other nations in protecting Americans and Europeans in Peking during a rebellion (called the Boxer Rebellion) that began in 1899, some Chinese branded the U.S. a foreign exploiter. Yet, after the war, the U.S. used some of the reparations that China paid to establish the “Boxer Indemnity Scholarship Fund,” an influential education program in China.
http://www.heritage.org/research/reports/2012/10/the-complicated-history-of-us-relations-with-china