The Jefferson Barracks Military Post is located on the Mississippi River at Lemay, Missouri, south of St. Louis.

It is the oldest operating U.S. military installation west of the Mississippi River, and it is now used as a base for the Army and Air National Guard.

General Henry Atkinson (commanding officer of the sixth infantry regiment), explorer William Clark, and Missouri Governor John Miller spent several days searching the banks of the Mississippi River for the perfect location for a new post to replace Fort Bellefontaine.

A site near the city of "Vide Poche" or Carondelet, ten miles (16 km) south of St. Louis, was recommended and then approved by Major General Jacob J.

On July 10, 1826, two days after the deed to the land was signed, the first military troops—six officers and 245 enlisted men of Companies A, B, H and I of the 1st Infantry Regiment, commanded by Brevet Major Stephen Watts Kearny—arrived at the new post and started building temporary quarters that they named Cantonment Miller in honor of Governor Miller.

Troops were deployed from Jefferson Barracks to push "hostile Indians" back into their village in present-day Iowa.

Jefferson Barracks became a major military post during the Mexican–American War (1846–1848) when it served as a rest and supply station for most U.S. troops deploying to Mexico.

[4][5] During the American Civil War (1861–1865), Jefferson Barracks served as a military hospital for both sides and a recruitment depot for the North.

During World War I, Jefferson Barracks served as a training and recruitment station for soldiers heading to Europe.

During World War II, Jefferson Barracks was a major reception center for U.S. troops being drafted into the military.

During World War II, Jefferson Barracks had a peak area of 1,518 acres (6.14 km2), and had billeting space for 16 officers and 1,500 enlisted persons.

Today, because of its large size, high school cross country races are often held in the park in the fall months, most notably the Hancock Invitational.

Additional galleries in the museum focus on the history of Jefferson Barracks, veterans organizations, and the Civil War in pop culture.

[11] The building housing the museum was constructed as a two-story duplex by Richard Deutman and Son architects and is one of two remaining structures that was part of Officers’ Row located on the north side of the former parade grounds.