The United States, having a vested economic interest in China, announced an Open Door policy in 1899. On 20 June, the Chinese Boxers began an eight-week campaign of rebellion and violence against foreigners, including the United States. Thereafter, the foreign powers together established a multinational force to combat the Boxers. On 14 August, this 19,000 strong military force, including the USA, stormed into Beijing and re-established order.
Richert, Lucas Paul. "The U.S. helped suppress Boxers in Beijing". The Literary Encyclopedia. First published 01 February 2009
[http://www.litencyc.com/php/stopics.php?rec=true&UID=5995, accessed 10 February 2016.]