The Reform War was a civil war in Mexico lasting from January 11, 1858 to January 11, 1861, fought between liberals and conservatives, over the promulgation of Constitution of 1857, which had been drafted and published under the presidency of Ignacio Comonfort. The constitution had codified a liberal program intended to limit the political, economic, and cultural power of the Catholic Church; separate church and state; reduce the power of the Mexican Army by elimination of the fuero; strengthen the secular state through public education; and economically develop the nation.

The first year of the war was marked by repeated conservative victories, but the liberals remained entrenched in the nation's coastal regions, including their capital at Veracruz giving them access to vital customs revenue.

Both governments attained international recognition, the Liberals by the United States, and the Conservatives by France, Great Britain, and Spain. Liberals negotiated the McLane–Ocampo Treaty with the United States in 1859. If ratified the treaty would have given the liberal regime cash but also would have granted the United States perpetual military and economic rights on Mexican territory. The treaty failed to pass in the U.S. Senate, but the U.S. Navy nonetheless helped protect Juárez's government in Veracruz.

Liberals thereafter accumulated victories on the battlefield until Conservative forces surrendered on December 22, 1860. Juárez returned to Mexico City on January 11, 1861 and held presidential elections in March. Although Conservative forces lost the war, guerrillas remained active in the countryside and would join the upcoming French intervention to help establish the Second Mexican Empire.