Alexander McSween (June 15, 1837 — July 19, 1878) was a prominent figure during the Lincoln County War of the Old West, and a central character, alongside John Tunstall, in opposing businessmen and gunmen Lawrence Murphy and James Dolan.

In 1875, the couple moved to Lincoln, New Mexico, where her husband had accepted a job as a lawyer for the mercantile company and bank of Irish Catholic immigrants Lawrence Murphy and James Dolan.

After he arrived in Lincoln County, New Mexico, however, McSween left Dolan and Murphy to work for English rancher and businessman John Tunstall, with whom he became close friends and, by 1876, business partners.

When the trouble between the two factions began, Murphy-Dolan accused McSween of embezzlement, and they hired gunmen from the Evans and Kinney gangs to rustle Tunstall's cattle and harass him.

On February 18, 1878, Tunstall was shot and killed, officially while resisting arrest, by Lincoln County Deputies William Morton, Jesse Evans, and Tom Hill.

On March 6, 1878, the Regulators arrested Frank Baker and Deputy William Morton, and subjected them both to summary execution On April 1, 1878, Billy the Kid, Jim French, Frank McNab, John Middleton, Fred Waite, Henry Brown, entered Lincoln and ambushed and murdered Lincoln County Sheriff William Brady and Deputy George Hindman.

Both Roberts and Regulator Dick Brewer were killed, Middleton was badly wounded, the Kid was grazed by a bullet, and George Coe had his trigger finger shot off.

Unlike with many other actual characters depicted in this movie and its sequel, McSween's death was portrayed more or less accurately, with a scene showing him shot and killed in the Battle of Lincoln.