Fort Halleck was a military outpost that existed in the 1860s along the Overland Trail and stage route in what was then the Territory of Idaho, now the U.S. state of Wyoming. The fort was established in 1862 to protect emigrant travelers and stages transporting mail between Kansas and Salt Lake City, Utah and named for Major General Henry Wager Halleck, commander of the Department of the Missouri and later General-in-chief of the Union armies.