Answer:
By the time of the Boxer Rebellion (1900-1901), United States was involved into his accelerated industrial growth and territorial expansion. In 1898, United States also was fighting against Spain for the possesion over Philippines and Puerto Rico.
First reason against the U.S. involvement in the Boxer Rebellion: By that time, the Monroe Doctrine had wide acceptance in the United States and in that context, the direct miltary involvement on foreign conflicts which involved third-party nations, was seen with reluctance. Since some sectors in United States wanted to set aside from the European and Japanes imperialist approach, some leaders suggested to not take direct part on the conflict.
Second reason: United Sates wanted to project an image of pro-democratic power, specially with its neighbors in Latin America. Therefore, its involvement in the conflict was relatively small. Rather, U.S. subscribed treaties to support Chinese students in order to project a different image set aside from the imperialists powers.
Explanation: