Final answer:
The Sioux and Apache posed the greatest threats to Texas settlers in the 1860s due to their effective warfare tactics. Brutal conflicts ensued with the settlers often backed by local militias and federal support, leading to forced relocation and significant changes to the Native American way of life.
Step-by-step explanation:
The Native American groups that posed the greatest threat to Texas settlers in the 1860s were primarily the Sioux in the Northern Plains and the Apache in the Southwest. The Sioux, under leaders like Red Cloud and Crazy Horse, were adept at high-speed mounted warfare, making them particularly significant opponents to frontier settlers. The Apache, renowned for their skills in desert and canyon warfare, also proved to be formidable opponents. During this period, American settlers and Native Americans were in constant conflict as settlers moved westward into territories that Native tribes had inhabited for centuries.
Conflicts between settlers and Native Americans were often brutal, involving local militias and eventually the support of the federal government. The goal of the settlers was to acquire land and eliminate Native tribes seen as obstacles, albeit the actual number of attacks by Native Americans was not proportional to the settlers' fear. In 1839, under President Mireau B. Lamar, Texas even adopted a policy of ethnic cleansing aimed at forcibly removing Native tribes from their lands. This era of western settlement was one that saw many Native tribes forcibly relocated or killed, with the U.S. government continuing policies of removal that began with the Indian Removal Act of 1830.
Despite some organized resistance from the tribes and occasional military victories, the result of these conflicts was profound changes in the American Indian way of life and substantial loss of tribal lands, leading to long-lasting consequences for Native American cultures. In the context of Texas, the expansion and conflict were part of a broader effort by white settlers—many of whom held racist views towards the Mexican government and native populations—to establish dominance and control over the region.