Civil war between white Americans and Native Americans in the 1870s and 1880s

By the time the Civil war between white Americans and Native Americans had largely ceased, the U.S. government had forced Indians out of fertile farmland in the Southeast and onto reservations in present-day Oklahoma in the 1830s, but most Americans assumed that the West, absent California, was a wasteland that whites would permanently avoid.

Given this, why did the United States-Indian warfare resume in the 1870s and 1880s, and why did this warfare take on such an apocalyptic scale?

What had changed, in other words, in United States-Indian relations, U.S. attitudes regarding the western frontier, American technology, military strategy, and so forth? Finally, what were the legacies of the last round of the United States-Indian warfare?