The film was loosely based on Harold Sinclair's 1956 novel of the same name, which in turn was based on the historic 17-day Grierson's Raid and Battle of Newton's Station in Mississippi during the Civil War.
In April 1863, Colonel Benjamin Grierson led 1,700 Illinois and Iowa soldiers from LaGrange, Tennessee to Baton Rouge, Louisiana, through several hundred miles of enemy territory, destroying Confederate railroad and supply lines between Newton's Station and Vicksburg, Mississippi. The mission was part of the Union Army's successful Vicksburg campaign to gain control over boat traffic on the Mississippi River, culminating in the Battle of Vicksburg. Grierson's destruction of Confederate-controlled rail links and supplies played an important role in disrupting Confederate General John C. Pemberton's strategies and troop deployments. Union General William Tecumseh Sherman reportedly described Grierson's daring mission as "the most brilliant of the war".
A Union cavalry brigade led by Colonel John Marlowe (John Wayne)—a railroad construction engineer in civilian life—is sent on a raid behind Confederate lines to destroy a railroad and supply depot at Newton Station. Major Henry Kendall (William Holden), a regimental surgeon who is torn between duty and the horror of war, is constantly at odds with Marlowe.
While the unit rests at Greenbriar Plantation, Miss Hannah Hunter (Constance Towers), the plantation's mistress, and her slave, Lukey (Althea Gibson) eavesdrop on a staff meeting as Marlowe discusses his battle strategy. To protect the secrecy of the mission, Marlowe is forced to take the two women with him. Initially hostile to her Yankee captor, Miss Hunter gradually warms to him. In addition to Kendall and Miss Hunter, Marlowe also must contend with Col. Phil Secord (Willis Bouchey), who continually second-guesses Marlowe's orders and command decisions.
Several battles ensue, including a fire fight during which Lukey is killed, and a skirmish with boy cadets from a local military school (based on the actual Battle of New Market). After destroying the crucial supply line, and with Confederate forces in pursuit, the brigade reaches a bridge that must be stormed in order to access the Union lines. Marlowe's men rig it with explosive charges, and Marlowe bids Hannah farewell. Kendall chooses to remain behind with some badly wounded men—knowing he will be captured with them—rather than leave them, defenseless, to the mercy of the Confederates. Marlowe, though wounded, lights the fuse and leads his troops across the bridge before it is destroyed, halting the Confederate advance. Their mission accomplished, he and his brigade continue on toward Baton Rouge.John Wayne as Colonel John MarloweWilliam Holden as Major Henry 'Hank' KendallConstance Towers as Miss Hannah Hunter of GreenbriarAlthea Gibson as Lukey, Miss Hunter's fiercely loyal black maidJudson Pratt as Sergeant Major KirbyKen Curtis as Cpl. WilkieWillis Bouchey as Col. Phil SecordBing Russell as Dunker, Yankee Soldier AmputeeO.Z. Whitehead as Otis 'Hoppy' Hopkins (medical assistant)Hank Worden as Deacon ClumpChuck Hayward as Union captainDenver Pyle as Jackie Jo (rebel deserter)Strother Martin as Virgil (rebel deserter)Basil Ruysdael as the Reverend (Jefferson Military Academy)Carleton Young as Col. Jonathan Miles, CSAWilliam Leslie as Maj. Richard GrayWilliam Henry as Confederate lieutenantWalter Reed as Union officerAnna Lee as Mrs. BufordWilliam Forrest as Gen. Steve HurlburtRon Hagerthy as BuglerRussell Simpson as Acting Sheriff Henry GoodbodyHoot Gibson as Sgt. BrownJack Pennick as Sgt. Maj.Mitch Mitchell (uncredited) Senior member of John Ford's Stock Company.
Exterior scenes were filmed in Natchitoches Parish, Louisiana, along the banks of Cane River Lake, and in and around Natchez, Mississippi. The film company built a bridge over the Cane River for the pivotal battle scene, and many locals were hired as extras.
The project was plagued from the start by cost overruns, discord, and tragedy. Holden and Ford argued incessantly. Wayne was preoccupied with pre-production logistics for The Alamo. Lukey's dialog was originally written in a stereotypic "Negro" dialect that Althea Gibson, the former Wimbledon and U.S. National tennis champion who was cast in the role, found offensive. She informed Ford that she would not deliver her lines as written. Though Ford was notorious for his intolerance of actors' demands, he agreed to modify the script.
During filming of the climactic battle scene, veteran stuntman Fred Kennedy suffered a broken neck while performing a horse fall and died. "Ford was completely devastated," wrote biographer Joseph Malham. "[He] felt a deep responsibility for the lives of the men who served under him." The film was scripted to end with the triumphant arrival of Marlowe's forces in Baton Rouge, but Ford "simply lost interest" after Kennedy's death. He ended the film with Marlowe's farewell to Hannah Hunter before crossing and blowing up the bridge.
The film was a commercial failure; analysts said this was due largely to Wayne's and Holden's high salaries, and the complex participation of multiple production companies. The response of audiences and critics was "lackluster".
Though based loosely on Grierson's Raid, The Horse Soldiers is a fictional account that departs considerably from the actual events. The real-life protagonist, a music teacher named Benjamin Grierson, becomes railroad engineer John Marlowe in the film. Hannah Hunter, Marlowe's love interest, has no historical counterpart. Numerous other details were altered as well, "to streamline and popularize the story for the non-history buffs who would make up a large part of the audience."