Fort Caspar is named for Caspar Collins, a lieutenant killed while trying to rescue a blockaded wagon train in 1865. Until that time the fort had been called Platte Bridge Station. Fort Caspar derives its primary significance from the concentration of many important events and activities. Early fur trade caravans passed this point regularly each season during the peak of trade in the region and used the ford to cross the North Platte. In addition, most travelers to California and Oregon from the 1840s through the 1860s traveled past this location and used the crossing. The transcontinental mail route passed here in 1851 and a stagecoach line used the route from 1858-1862. The Fort was established in order to protect the Platte River Bridge which was built in 1858 and used until late 1867. The Army garrisoned the site from 1858-1859 and again from 1862-1867.
The dominant structure throughout the 1858-1867 period was Guinard's Platte Bridge. Louis Guinard had a small residence and store near the south end of the bridge. This was expanded to house a telegraph station in 1861. The precise nature and location of the 1858-1859 army buildings are not known. However, ground plans and drawings cover the 1863 period structures which housed the telegraph station garrison. These, along with archaeological data, formed the basis for the reconstruction of the 1863 structures completed in 1939. During 1865-1867 the army expanded the post considerably. Excellent plans and elevations along with materials lists and specifications cover this construction period. The buildings were of log construction, with plank floors and roofs of puncheons, covered with clay. At its peak of development the post included over two dozen substantial buildings and additional lesser outbuildings, and housed from three hundred to four hundred men, one of the largest garrisons in the West.