It seems by cooperating with the US government, https://en.wikipedia.org/wiki/Cahuilla#History:

“During the Mexican–American War, Juan Antonio led his warriors to join Californios led by José del Carmen Lugo in attacking their traditional enemy, the Luiseño. Lugo led this action in retaliation for the Pauma Massacre, in which the Luiseno had killed 11 Californios. The combined forces staged an ambush and killed 33–40 Luiseno warriors, an event that became known as the Temecula Massacre of 1847. (Historians disagree on the exact number of deaths; Luiseno oral tradition holds that more than 100 warriors were killed.) In the treaty ending the war with Mexico, the US promised to honor Mexican land grants and policies. These included recognition of Native American rights to inhabit certain lands, but European-American encroachment on Indian lands became an increasing problem after the US annexed California.

[…]

In addition to the influx of Anglo-American miners, ranchers and outlaws, and groups of Mormon colonists, the Cahuilla came into conflict with the neighboring Cupeño tribe to the west. In November 1851, the Garra Revolt occurred, wherein the Cupeno leader Antonio Garra attempted to bring Juan Antonio into his revolt. Juan Antonio, friendly to the Americans, was instrumental in capturing Garra, ending that revolt. When the California Senate refused to ratify an 1852 treaty granting the Cahuilla control of their land, some tribal leaders resorted to attacks on approaching settlers and soldiers. Juan Antonio did not participate in this as long as he lived.

To encourage the railroad, the U.S. government subdivided the lands into one-mile-square sections, giving the Indians every other section. In 1877 the government established reservation boundaries, which left the Cahuilla with only a small portion of their traditional territories.”