Long before the Army arrived, this bend in the North Platte River served as a natural gateway. Brigham Young commissioned a ferry here in 1847, manned by nine Mormons using cottonwood dugout canoes lashed together with planking. The crossing was so vital that Mormon operators returned every summer until 1852, eventually replacing the ferry with a rope-and-pulley system that could haul wagons across in five minutes. The Pony Express ran a station here from 1860 to 1861. Louis Guinard, a French Canadian entrepreneur, built a toll bridge at the site in 1859, creating Platte Bridge Station. His 1,000-foot span of pine planks and cottonwood pilings represented the only reliable crossing for hundreds of miles. Every wagon train headed to Oregon, California, or Utah Territory passed through this bottleneck.

By 1865, Guinard's bridge had made him prosperous and made the Army nervous. The telegraph line now ran alongside the Oregon Trail, and protecting both the emigrants and the wire required soldiers stationed at the crossing. But the soldiers at Platte Bridge Station that July were not sheltered behind the small trading post stockade with its 14-foot pine logs. They were camped in tents, exposed on the open plain. Lakota, Cheyenne, and Arapaho scouts noted this vulnerability. Survivors of the Sand Creek Massacre, where Colorado militia had killed over 150 Cheyenne the previous November, were among those who gathered north of the river. On July 25, 1865, a force estimated at several thousand warriors approached from the bluffs.

Lieutenant Caspar Collins was 20 years old, the son of the colonel commanding Fort Collins in Colorado. On the morning of July 26, Major Martin Anderson ordered Collins and 25 men to ride across the bridge and rescue an approaching wagon train. Everyone at the station could see warriors massing on the surrounding hills. Collins crossed the Platte Bridge at a walk, formed his men into a column of fours, and rode west at a trot. Hundreds of Cheyenne emerged from sand hills and gullies. The concealed force may have numbered a thousand. The fight lasted only minutes. Collins was killed. Five miles west, Sergeant Amos Custard's wagon train was attacked within sight of the station. All 22 soldiers with the wagons died. The Army officially counted 28 dead. The Lakota and Cheyenne lost perhaps 60 killed and 130 wounded in fighting around the station that week.

The Army renamed the post Fort Caspar in August 1865, using Lieutenant Collins' first name to distinguish it from his father's fort in Colorado. Two years later, the garrison transferred to the newly built Fort Fetterman, and the Lakota burned Guinard's bridge behind them. Louis Guinard the elder had already died, having fallen from his own bridge and drowned just weeks before the battle. His nephew spent 25 years in court trying to recover compensation for the destroyed property, but the will proving his inheritance had been lost at Fort Laramie. He never saw a dollar. In 1936, the City of Casper reconstructed the fort using sketches Lieutenant Collins had drawn in 1863, preserving the post as it appeared during the years leading up to that fatal morning.