It is pretty interesting to see how many famous commanders from the Civil War served in the West, in the Mexican War or the Indian Wars.
Camp Cooper was officially closed during the departure of all Federal troops from Texas forts and other facilities after the Texas Secession Convention adopted the Texas Ordinance of Secession on February 1, 1861. General Twiggs surrendered the Federal forts and facilities in Texas on February 16, 1861. Camp Cooper was officially abandoned on February 21, 1861.
From The Handbook of Texas by the Texas State Historical Association:
Camp Cooper was on the Clear Fork of the Brazos River seven miles north of the site of present Fort Griffin State Historic Site in south central Throckmorton County. It was established by the Texas legislature in January 1856 and named for United States Army adjutant general Samuel Cooper. Its mission was to protect the frontier and to monitor the nearby Comanche Indian reservation. The area had been a campsite for three companies of the Fifth Infantry in 1851. The site was subsequently surveyed by Capt. Randolph B. Marcy and Robert S. Neighbors. The post was founded by Col. Albert Sidney Johnston in January 1856 and became headquarters for four companies of the Second United States Cavalry under the command of Lt. Col. Robert E. Lee. This was Lee's first command of a fort. He remained in charge for fifteen months, from April 9, 1856, until July 22, 1857. Captains under his command included Earl Van Dorn and Theodore O'Hara.
Although the camp initially had adequate military stores, it was plagued by severe weather, insects, dust, and irregular supply trains. Rattlesnakes were constant visitors, and Lee kept one as a pet. When he left the camp in 1857 for San Antonio, Maj. George H. Thomas took over. Thomas commanded the Cimarron expedition into Northwest Texas that same year. Troops from Camp Cooper participated in numerous campaigns and police actions against hostile Indians, including the pursuit of Peta Nocona's Comanches that resulted in the death of Peta Nocona and the recapture of Cynthia Ann Parker. Local unrest declined after 1859, when the Comanche reservation was dissolved and the Indians were removed from the area. ...
The people on the Texas frontier had been asking for some time that the Comanche Reservation be closed and moved to Indian Territory (Oklahoma). When the Indians were moved, the number of raids on settlers decreased and there was less need for the camp. [Info about the Comanche Reservation]
On the other hand, after the Federal troops left and Texas men joined Texas troop units or the Confederate Army and went to fight outside of the state, the frontier was not well defended and the edge of the frontier moved back 200 miles to the east in the face of increasing Indian raids. One of my great-great grandfathers served in a Texas Ranger frontier unit during that time. Another Texas ancestor, my great grandfather from another family, was a boy during the Civil War. I knew him when I was a boy. He remembered Indian raids in his Texas county in the 1860s.