Antonio López de Santa Anna in the context of "Martín Perfecto de Cos"

The First Mexican Republic, known also as the First Federal Republic (Spanish: Primera República Federal), existed from 1824 to 1835. It was a federated republic, established by the Constitution of 1824, the first constitution of independent Mexico, and officially designated the United Mexican States (Spanish: Estados Unidos Mexicanos, listen). It ended in 1835, when conservatives under Antonio López de Santa Anna transformed it into a unitary state, the Centralist Republic of Mexico.

The republic was proclaimed on 1 November 1823 by the Supreme Executive Power, months after the fall of the Mexican Empire ruled by emperor Agustin I, a former royalist military officer-turned-insurgent for independence. The federation was formally and legally established on 4 October 1824, when the Federal Constitution of the United Mexican States came into force.

The Mexican–American War, also known in the United States as the Mexican War and in Mexico as the United States intervention in Mexico, (April 25, 1846 – February 2, 1848) was an invasion of Mexico by the United States. It followed the 1845 American annexation of Texas, which Mexico still considered its territory because it refused to recognize the Treaties of Velasco, signed by President Antonio López de Santa Anna after he was captured by the Texian Army during the 1836 Texas Revolution. The Republic of Texas was de facto an independent country, but most of its Anglo-American citizens who had moved from the United States to Texas after 1822 wanted to be annexed by the United States.

Sectional politics over slavery in the United States had previously prevented annexation because Texas would have been admitted as a slave state, upsetting the balance of power between Northern free states and Southern slave states. In the 1844 United States presidential election, Democrat James K. Polk was elected on a platform of expanding U.S. territory to Oregon, California (also a Mexican territory), and Texas by any means, with the 1845 annexation of Texas furthering that goal. However, the boundary between Texas and Mexico was disputed, with the Republic of Texas and the U.S. asserting it to be the Rio Grande and Mexico claiming it to be the more-northern Nueces River. Polk sent a diplomatic mission to Mexico in an attempt to buy the disputed territory, together with California and everything in between for $25 million (equivalent to $798 million in 2024), an offer the Mexican government refused. Polk then sent a group of 80 soldiers across the disputed territory to the Rio Grande, ignoring Mexican demands to withdraw. Mexican forces interpreted this as an attack and repelled the U.S. forces on April 25, 1846, a move which Polk used to convince the Congress of the United States to declare war.

In the history of Mexico, La Reforma (from Spanish: "The Reform"), or reform laws, refers to a pivotal set of laws, including a new constitution, that were enacted in the Second Federal Republic of Mexico during the 1850s after the Plan of Ayutla overthrew the dictatorship of Santa Anna. They were intended as modernizing measures: social, political, and economic, aimed at undermining the traditional power of the Catholic Church and the army. The reforms sought separation of church and state, equality before the law, and economic development. These anticlerical laws were enacted in the Second Mexican Republic between 1855 and 1863, during the governments of Juan Álvarez, Ignacio Comonfort and Benito Juárez. The laws also limited the ability of Catholic Church and Indigenous communities from collectively holding land. The liberal government sought the revenues from the disentailment of church property, which could fund the civil war against Mexican conservatives and to broaden the base of property ownership in Mexico and encouraging private enterprise. Several of them were raised to constitutional status by the constituent Congress that drafted the liberal Constitution of 1857. Although the laws had a major impact on the Catholic Church in Mexico, liberal proponents were not opposed to the church as a spiritual institution, but rather sought a secular state and a society not dominated by religion.

The Juárez Law reduced the power that military and ecclesiastical courts held. The Lerdo Law forced land held in collective ownership to be sold to individual owners. It aimed at creating a dynamic real estate market, creating a class of yeoman farmers owning their own land, and raising revenue for the state. The measure was intended to strip the Church of most of its property, as well as to break Indigenous communities' collective ownership of land.

The Reform War (17 December 1857 – 11 January 1861) or War of Reform (Spanish: Guerra de Reforma), also known as the Three Years' War (Spanish: Guerra de los Tres Años), and the Mexican Civil War, was a complex civil conflict in Mexico fought between Mexican liberals and conservatives with regional variations over the promulgation of Constitution of 1857. It has been called the "worst civil war to hit Mexico between the War of Independence of 1810–21 and the Revolution of 1910–20". Following the liberals' overthrow of the dictatorship of conservative Antonio López de Santa Anna, liberals passed a series of laws codifying their political program. These laws were incorporated into the new constitution. It aimed to limit the political power of the executive branch, as well as the political, economic, and cultural power of the Catholic Church. Specific measures were the expropriation of Church property; separation of church and state; reduction of the power of the Mexican Army by elimination of their special privileges; strengthening the secular state through public education; and measures to develop the nation economically.

The constitution had been promulgated on 5 February 1857 was to come into force on 16 September 1857. There was intense opposition from Conservatives and the Catholic Church over its anti-clerical provisions, but there were also moderate liberals, including President Ignacio Comonfort, who considered the constitution too radical and likely to trigger a civil war. The Lerdo Law forced the sale of most of the Church's rural properties. The measure was not exclusively aimed at the Catholic Church, but also Mexico's indigenous peoples, which were forced to sell sizeable portions of their communal lands. Controversy was further inflamed when the Catholic Church decreed the excommunication of civil servants who took a government-mandated oath upholding the new constitution, which left Catholic civil servants with the choice of losing their jobs or being excommunicated.

José de la Cruz Porfirio Díaz Mori (/ˈdiːəs/; Spanish: [poɾˈfiɾjo ˈði.as]; 15 September 1830 – 2 July 1915) was a Mexican general and politician who was the dictator of Mexico from 1876 until his overthrow in 1911, seizing power in a military coup. He served on three separate occasions as President of Mexico, a total of over 30 years, this period is known as the Porfiriato and has been called a de facto dictatorship. Díaz’s time in office is the longest of any Mexican ruler.

Díaz was born to a Oaxacan family of modest means. He initially studied to become a priest but eventually switched his studies to law, and among his mentors was the future President of Mexico, Benito Juárez. Díaz increasingly became active in Liberal Party politics fighting with the Liberals to overthrow Santa Anna in the Plan of Ayutla, and also fighting on their side against the Conservative Party in the Reform War.

The Centralist Republic of Mexico (Spanish: República Centralista de México), or in the anglophone scholarship, the Central Republic, officially the Mexican Republic (Spanish: República Mexicana), was a unitary political regime established in Mexico on 23 October 1835, under a new constitution known as the Siete Leyes (lit. 'seven laws') after conservatives repealed the federalist Constitution of 1824 and ended the First Mexican Republic. It would ultimately last until 1846, when the Constitution of 1824 was restored at the beginning of the Mexican–American War.

Two presidents would predominate throughout this era: Santa Anna and Anastasio Bustamante.

The Treaties of Velasco were two documents, one private and the other public, signed in Fort Velasco on May 14, 1836 between General Antonio López de Santa Anna and the Republic of Texas in the aftermath of the Battle of San Jacinto on April 21, 1836. The part of the former Velasco, Texas, in which the fort was located is now part of the present-day location of Surfside Beach. The signatories were Interim President David G. Burnet for Texas and Santa Anna for Mexico. Texas intended the agreements to conclude hostilities between the two armies and offer the first steps toward the official recognition of Texas's independence from Mexico.

Santa Anna had been captured by Texans. Many wanted him hanged as just vengeance for the murder of comrades and family during the battles at Alamo and Goliad, but Sam Houston spared his life and extracted a promise from Santa Anna that Mexican troops would be removed from Texas. Santa Anna persuaded Burnet that if he were allowed to return to Mexico City, he would argue for the independence of Texas. Santa Anna signed both the public agreement and the secret treaty, but neither was ratified as a treaty by the Mexican government because he had signed them under duress while being held captive. Santa Anna later said, "I did promise to try to get a hearing for the Texas Commissioners, but this in itself did not bind the government to receive them."

The Texas Revolution (October 2, 1835 – April 21, 1836) was a rebellion by Anglo-American immigrants as well as Hispanic Texans (known as Texians and Tejanos respectively) against the centralist government of Mexico in the Mexican state of Coahuila y Tejas. Although the uprising was part of a larger revolt against the Centralist Republic of Mexico that included other provinces opposed to the regime of President Antonio López de Santa Anna, the Mexican government believed the United States had instigated the Texas insurrection with the goal of annexation. The Mexican Congress passed the Tornel Decree, declaring that any foreigners fighting against Mexican troops "will be deemed pirates and dealt with as such, being citizens of no nation presently at war with the Republic and fighting under no recognized flag". Only the province of Texas succeeded in breaking with Mexico, establishing the Republic of Texas. It was eventually annexed by the United States about a decade later.

The Republic of Texas (Spanish: República de Tejas), or simply Texas, was a sovereign country in North America from March 2, 1836, to February 19, 1846. Texas was bordered by Mexico to the west and southwest, the Gulf of Mexico to the southeast, the two U.S. states of Louisiana and Arkansas to the east and northeast, and U.S. unorganized territory encompassing parts of the current U.S. states of Oklahoma, Kansas, Colorado, and Wyoming to the north. The Texas Revolution began when hostilities broke out on October 2, 1835, shortly before the regime of Mexican President and General Antonio López de Santa Anna adopted a new Mexican constitution known as the Siete Leyes that abolished the authority of the states under the federal republic and established a centralized government. The revolution lasted for over six months. On March 2, 1836, delegates in convention proclaimed the Texas Declaration of Independence. Major fighting ended on April 21, 1836, with the Treaties of Velasco that ended the fighting and secured Texas' independence.

The Mexican Congress refused to recognize the independence of the Republic of Texas, as the Treaties of Velasco were signed by Santa Anna under duress as prisoner. The majority of the Mexican Congress did not approve the agreement. Much of its territory was controlled and disputed by Mexico or the Comancheria; Mexico considered it a rebellious province during its entire existence. The Anglo-American residents of the area and of the republic were referred to as Texians, while Texans of Mexican descent were referred to as Tejanos.