Short answers, pulled from the story.
Philip Henry Sheridan was a Union general in the American Civil War who rose from the rank of captain to major general in six months and became one of the highest-ranking officers in U.S. Army history. He is famous for his Shenandoah Valley campaigns in 1864, including the destruction known as "The Burning", his role in forcing Robert E. Lee's surrender at Appomattox Court House in 1865, and his advocacy for Yellowstone National Park.
At Appomattox Court House on the 9th of April, 1865, Sheridan's cavalry blocked Lee's escape route, forcing the surrender of the Army of Northern Virginia. President Lincoln had telegraphed Grant on April 7 quoting Sheridan: "If the thing is pressed I think that Lee will surrender."
"The Burning" was the name given by Valley residents to Sheridan's systematic destruction of the Shenandoah Valley's agricultural infrastructure in the fall of 1864. Sheridan's forces burned barns, mills, factories, and railroads, seized or killed livestock, and rendered over 400 square miles uninhabitable as part of a scorched-earth strategy ordered by General Grant.
Sheridan denied making the statement. Biographer Roy Morris Jr. notes that the quote first appeared in print more than a hundred years after the alleged 1869 exchange, in Dee Brown's 1970 book Bury My Heart at Wounded Knee. Sheridan's own recorded words include acknowledging that the U.S. had taken the tribes' land and means of support, and blaming Congress for failing to provide adequate food to reservation populations.
Sheridan lobbied Congress to prevent a railroad and land-development scheme in Yellowstone after the Department of the Interior granted development rights in 1882. He arranged a presidential expedition for Chester A. Arthur and secured a rider to the Sundry Civil Bill of 1883 that provided military control and sharply limited development. In 1886 he ordered the 1st U.S. Cavalry into the park, and the military operated Yellowstone until the National Park Service took it over in 1916.
Sheridan died on the 5th of August, 1888, holding the rank of General of the Army, the same rank previously held by Ulysses S. Grant and William T. Sherman. Congress passed the legislation promoting him on the 1st of June 1888, while he was suffering from heart attacks, and he received the news from a congressional delegation shortly before his death.