The main consequence of the Boxer Rebellion in 1900-01 was that China was greatly weakened and controlled to an even greater extent by the western imperial powers. Those empires did, however, decide as a result of the rebellion that attempting to make China a colony was probably a bad idea.
The rebellion itself was an uprising against those same western influences and specifically against the Christian religion. This led to another important and tragic consequence, the genocide of more than a million Chinese Christians during the uprising.
The United States took part in the international effort to put down the rebellion, and this was the point at which the new American empire entered the competition for Asian trade, colonies and resources. Secretary of State John Hay issued his Open Door Note "suggesting" that American commercial interests in China would be protected and nudging America's way into line next to (and ahead of) the other empires.
Japan also gained recognition for its military aid to the suppression effort and Russia gained some territory in Manchuria.