On January 7th, 1865, Camp Rankin and its assigned troops became central players in the Cheyenne, Arapaho, and Sioux response to the Sand Creek Massacre. That morning the Native American warriors began a sustained six-week offensive operation to interdict the Oregon Trail along the North Platte River and the Denver Road along the South Platte River. Their campaign centered on sacking Julesburg station located a mile east of the post. However, the assembled warriors also attacked stage stations, ranches, wagon trains, and the transcontinental telegraph over nearly 150-miles of the two trails. The tribes returned to Julesburg station again on February 2nd to sack and burn the station one more time before heading north towards the Black Hills.
The post was renamed Fort Sedgwick in September 1865 after Major General John Sedgwick who was killed at the Battle of Spotsylvania Court House on May 9, 1865. The exact rational behind the name change is likely related to Major General Sedgwick's service as a major under Colonel Edwin Vose Sumner during the First Dragoon's 1857 expedition against the Cheyenne. During the expedition, Sedgwick led one column through the area that became eastern Colorado.1.
After the conclusion of the Civil War, regular Army troops returned to the west. Their return brought with them inspections from the War Department that determined the post was unfit for troops. An expansion and modernization program was begun in 1866 and the post continually grew over the next few years. Its expansion was directly tied to the progress of the transcontinental railroad across Nebraska as the troops at the post were assigned to provide security to the railroad's construction crews. They also continued to provide security to the wagon trains, stage coaches, and emigrant trains that continued across the Overland Trail. Soldiers from the post were also involved in the continued stability operations against the Sioux, Cheyenne, and Arapaho in northeastern Colorado until the forced removal of the Cheyenne and Arapaho from Colorado in 1869.
The post became increasingly challenged to support its missions due to its location on the south side of the Platte River. The river adjacent the fort was wide and often impassable by fording. As a result, as the post had to defend the railroad on the north side of the river, the decision was made in 1866 to open a sub-installation, Camp Sidney, 45 miles up stream Lodgepole Creek to provide a base of operations for the troops that was not dependent on their ability to ford the South Platte. Eventually, Camp Sidney became its own post and its location rendered Fort Sedgwick unneeded. In 1871, with the transcontinental railroad across the states, the Denver Pacific from Denver to Cheyenne, and the Kansas Pacific from Kansas City to Denver completed, Fort Sedgwick was closed.