There were at least two groups against whom Captain Jack and the Modocs struggled in Chapter 10.
First, the Modocs struggled against other Indians. Specifically, they struggled against the Klamath Indians. This struggle came about because the Modocs were put on a reservation in Oregon on land that the Klamaths felt was theirs.
Second, the Modocs struggled against the US Army. The Modocs left the reservation in Oregon and moved back across the border to their own lands in Northern California. There, they came into conflict with the Army when it tried to get them to return to the reservation. This led to the Modoc War in 1872-3, which ended in what is now Lava Beds National Monument in California.