The Thousand Days' War was a three-year-long civil war fought between the conservative government of Colombia and the rebellious Colombian Liberal Party. The war resulted in a Conservative victory and the confirmation of Colombia's unitary political structure. However, Panama was able to break away from Colombia with American assistance, and up to 180,000 Colombians (3.8% of the population) was killed.
Throughout the 19th century, Colombia was polarized between conservative and liberal political forces which pushed for a unitary state and a federalized state, respectively. Following a civil war in 1885, a new constitution began a centralist political process known as the Regeneracion, which brought peace to Colombia while failing to resolve the internal political and economic disputes between the disparate departments of the country. Tensions ran high after the presidential election of 1898, and, on 17 October 1899, members of the Colombian Liberal Party in Santander department announced an official insurrection against the government.
In Colombia, the Colombian Liberal Party drew support primarily from the middle and lower classes, including rural workers, peasants, and some urban workers. Intellectuals and progressive elements also leaned towards the Liberals, advocating for social reforms and greater political participation. The Liberals were particularly strong in the western regions of Colombia, especially in departments like Valle del Cauca and Antioquia. They also had significant support in the coffee-growing areas. The Colombian Conservative Party was backed by the elite, landowners, and the Catholic Church, representing the interests of the wealthy classes and rural aristocracy. Conservatives had strong support in the eastern and southern regions, particularly in departments like Boyaca and Cundinamarca. They were also influential in rural areas where landownership was concentrated. The National Party, a coalition of both Liberals and Conservatives, attracted those who were disillusioned with the extremes of both parties. Their support came from various social strata, including moderates and those seeking stability. They did not have a clearly defined geographical base, instead seeking to unify various factions and being active in areas where both Liberal and Conservative tensions were prevalent.
Hostilities began with the Battle of Bucaramanga, when liberal factions attempted to take over the city starting on 11 November 1899. The Conservative government responded by dispatching forces to the city, defeating the Liberals at the Battle of the Magdalena River on 24 October 1899. The sickly President Manuel Antonio Sanclemente was deposed by his own Conservative Party and was replaced by Jose Manuel Marroquin. Bloody battles were fought at Peralonso and Palonegro in 1900, and, after the Liberal defeat at the Battle of Palonegro, both the Liberal and Conservative parties split into peace and war factions. The National faction of the Conservatives supported an end to the war, which had become confined to Panama and the Caribbean coast, causing the government to avoid internalizing the war. Liberal governments in neighboring Latin American countries attempted to support the Liberal rebellion, but Marroquin's victory over the Liberals at the Battle of San Cristobal in July 1901 reduced Venezuelan aid to the Liberals. However, the Venezuelan Army was sent into Colombia that September, only to meet with defeat and a mass retreat following the Battle of Riohacha. Eventually, the Liberal general Rafael Uribe Uribe expressed an inclination towards a conditional surrender, and the first peace treaty was signed at Neerlandia on 24 October 1902. At the same time, the US Navy was sent to Panama to protect America's future interests in the construction of the Panama Canal, forcing the Liberals there to surrender. On 21 November 1902, the definitive peace treaty was signed aboard the American battleship USS Wisconsin. The Conservatives would dominate Colombian politics for another 28 years, although they ruled over a nation with a partially collapsed economy and continued governmental instability.