Answer by taking a quote from the following article:

Philip Henry Sheridan (March 6, 1831 - August 5, 1888) was a career United States Army officer and a Union general in the American Civil War. His career was noted for his rapid rise to major general and his close association with General-in-chief Ulysses S. Grant, who transferred Sheridan from command of an infantry division in the Western Theater to lead the Cavalry Corps of the Army of the Potomac in the East. In 1864, he defeated Confederate forces in the Shenandoah Valley and his destruction of the economic infrastructure of the Valley, called "The Burning" by residents, was one of the first uses of scorched earth tactics in the war. In 1865, his cavalry pursued Gen. Robert E. Lee and was instrumental in forcing his surrender at Appomattox.

The protection of the Great Plains fell under the Department of the Missouri, an administrative area of over 1,000,000 mi.2, encompassing all land between the Mississippi River and the Rocky Mountains. Maj. Gen. Winfield Scott Hancock was assigned to the department in 1866 but had mishandled his campaign, resulting in Sioux and Cheyenne raids that attacked mail coaches, burned the stations, and killed the employees. They also killed and kidnapped a considerable number of settlers on the frontier. Under pressure from the governors, General Grant turned to Sheridan. In September 1866, Sheridan arrived at the former Fort Martin Scott near Fredericksburg, Texas, where he spent three months subduing Indians in the Texas Hill Country.  In August 1867, Grant appointed Sheridan to head the Department of the Missouri and pacify the Plains. His troops, even supplemented with state militia, were spread too thin to have any real effect. He conceived a strategy similar to the one he used in the Shenandoah Valley. In the Winter Campaign of 1868-69 (of which the Battle of Washita River was part) he attacked the Cheyenne, Kiowa, and Comanche tribes in their winter quarters, taking their supplies and livestock and killing those who resisted, driving the rest back into their reservations. When Sherman was promoted to General of the Army following Grant's election as President of the United States, Sheridan was appointed to command the Military Division of the Missouri, with all the Great Plains under his command. Professional hunters, trespassing on Indian land, killed over 4 million bison by 1874, and Sheridan applauded: "Let them kill, skin and sell until the buffalo is exterminated". When the Texas legislature considered outlawing bison poaching on tribal lands, Sheridan personally testified against it, suggesting that the legislature should give each of the hunters a medal, engraved with a dead buffalo on one side and a discouraged-looking Indian on the other.  Eventually the Indians returned to their designated reservations. Sheridan's department conducted the Red River War, the Ute War, and the Great Sioux War of 1876-77, which resulted in the death of a trusted subordinate, Lt. Col. George Armstrong Custer. The Indian raids subsided during the 1870s and were almost over by the early 1880s, as Sheridan became the commanding general of the U.S. Army.  Comanche Chief Tosawi reputedly told Sheridan in 1869, "Tosawi, good Indian," to which Sheridan supposedly replied, "The only good Indians I ever saw were dead." In Bury My Heart at Wounded Knee, Dee Brown attributed the quote to Sheridan, stating that "Lieutenant Charles Nordstrom, who was present, remembered the words and passed them on, until in time they were honed into an American aphorism: The only good Indian is a dead Indian. Sheridan denied he had ever made the statement. Biographer Roy Morris Jr. states that, nevertheless, popular history credits Sheridan with saying "The only good Indian is a dead Indian." This variation "has been used by friends and enemies ever since to characterize and castigate his Indian-fighting career."

What happened as a result of this?
killing those who resisted, driving the rest back into their reservations.