In 1861, volunteer cavalry were ordered to Guinard’s Bridge to guard against the increasingly frequent Indian Raids. The following year, the trading post became a one-company military post and was officially renamed Platte Bridge Station.
In response to the 1864 Sand Creek Massacre of Black Kettle's Cheyenne by Colonel Chivington’s militia in Colorado Territory, Plains tribes increased raids along the trails the following spring. In Sioux and Cheyenne warriors, including Red Cloud, Old Man Afraid of His Horse Roman Nose, and Dull Knife, began to threaten the bridge. On July 26th, when an eastern bound Army wagon train was due to come in, Lieutenant Caspar Collins led his men to drive off the hostile Indians. As Collins and his troops crossed the bridge, they were quickly driven back by the Indians and in the battle that ensued, Collins and four other soldiers were killed. The battle became known as the Battle of Platte Bridge Station.
Troops enlarged and rebuilt the fort in 1866, but when the the Union Pacific Railroad and the new transcontinental telegraph reached Cheyenne, Wyoming in the fall of 1867, migration along the the Oregon-California-Mormon Pioneer Trail dramatically began to wane. The army then began to establish new installations to protect the railroad route across southern Wyoming. On October 19, 1867, orders were issued to abandon Fort Casper and troops and materials, including some of the buildings were transferred to Fort Fetterman, Wyoming. Almost immediately after the troops were gone, the Indians burned the buildings and the bridge. What was left quickly fell into ruins and by the 1870s the site of the old post became part of the CY Ranch. In 1936, Casper citizens and the Works Progress Administration (WPA) reconstructed parts of the fort and the Platte Bridge Station using sketches made by Caspar Collins and others in the 1860s. In the 1980s, a replica of the Mormon ferry and a reconstructed section of the Guinard bridge were added to the grounds. http://www.legendsofamerica.com/wy-fortcaspar.html
Louis Guinard constructed a bridge at the site of the Mormon Ferry in 1859, with the first migration crossing in 1860. This bridge was nearly one thousand feet long and seventeen feet wide. It was supported by 28 stone-filled cribs and cost nearly $40,000 to build. It competed with Reshaw’s Second Bridge for the remainder of the emigration, charging between $1 and $6 depending on river conditions. The site, originally known as Mormon Ferry , became Platte Bridge Station and then Fort Caspar. Abandoned in 1867, "all salvagable material" was used to build Fort Fetterman. http://wyoshpo.state.wy.us/trailsdemo/platteriverfords.htm