The Lincoln County War was an Old West conflict between rival factions which began in 1878 in New Mexico Territory, the predecessor of the state of New Mexico, and continued until 1881. The feud became famous because of the participation of William H. Bonney ("Billy the Kid"). Other notable participants included Sheriff William J. Brady, cattle rancher John Chisum, lawyer and businessman Alexander McSween, James Dolan and Lawrence Murphy.The conflict began between two factions competing for profits from dry goods and cattle interests in the county. The older, established faction was dominated by James Dolan, who operated a dry goods monopoly through a general store referred to locally as "The House". English-born John Tunstall and his business partner Alexander McSween opened a competing store in 1876, with backing from established cattleman John Chisum. The two sides gathered lawmen, businessmen, Tunstall's ranch hands, and criminal gangs to their assistance. The Dolan faction was allied with Lincoln County Sheriff Brady and aided by the Jesse Evans Gang. The Tunstall-McSween faction organized their own posse of armed men, known as the Lincoln County Regulators, and had their own lawmen consisting of town constable Richard M. Brewer and Deputy US Marshal Robert A. Widenmann.The conflict was marked by revenge killings, starting with the murder of Tunstall by members of the Evans Gang. In revenge for this, the Regulators killed Sheriff Brady and others in a series of incidents. Further killings continued unabated for several months, climaxing in the battle of Lincoln, a five-day gunfight and siege that resulted in the death of McSween and the scattering of the Regulators. Pat Garrett was named County Sheriff in 1880, and he hunted down Billy the Kid, killing two other former Regulators in the process.

The war was fictionalized by several Hollywood movies, including The Left Handed Gun in 1958, John Waynes Chisum in 1970, Sam Peckinpahs Pat Garrett and Billy the Kid in 1973 and Young Guns in 1988. Ron Hansens novel The Kid (2016) is also inspired by the Lincoln County War.

John Henry Tunstall (6 March 1853 – 18 February 1878) was an English-born rancher and merchant in Lincoln County, New Mexico, United States. He competed with the Irish Catholic merchants, lawmen, and politicians who ran the town of Lincoln and the county. Tunstall, who hoped to unseat the Irish and make a fortune as the County's new boss, was the first man killed in the Lincoln County War, an economic and political conflict that resulted in armed warfare, between rival gangs of cowboys and the ranchers, lawmen, and politicians who issued the orders.