Agustín de Iturbide in the context of "Treaty of Córdoba"

The Mexican Empire (Spanish: Imperio Mexicano, pronounced [imˈpeɾjo mexiˈkano] ) was a constitutional monarchy and the first independent government of Mexico. It was also the only former viceroyalty of the Spanish Empire to establish a monarchy after gaining independence. The empire existed from 1821 to 1823, making it one of the few modern-era independent monarchies in the Americas. To distinguish it from the later Second Mexican Empire (1864–1867) under Emperor Maximilian, this historical period is commonly referred to as the First Mexican Empire. The empire was led by former Royal Spanish military officer Agustín de Iturbide, who ruled as Agustín I.

The establishment of a monarchy was the initial goal for an independent Mexico, as outlined in the Plan of Iguala, a political document drafted by Iturbide that unified the forces fighting for independence from Spain. Following the signing of the Treaty of Córdoba by the last Spanish viceroy in September 1821, the plan for a Mexican monarchy advanced. Iturbide's popularity reached its peak on May 18, 1822, when public demonstrations called for him to become emperor in the absence of a European royal willing to assume the throne. The Mexican Congress approved the proposal, and Iturbide was crowned in July 1822.

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.

Monarchism in Mexico is the political ideology that defends the establishment, restoration, and preservation of a monarchical form of government in Mexico. Monarchism was a recurring factor in the decades during and after Mexico's struggle for independence.

Beginning in 1808, it was unclear near the ending of the kingdom of the Viceroyalty of New Spain what form of government—monarchical or republican—might replace the absolutism of Ferdinand VII of Spain, but the default position in that era was monarchy. In 1821, Mexico declared the Independence of the Mexican Empire. However, lacking a prince to ascend the Throne of Mexico, Agustín de Iturbide, a criollo royalist general who made an alliance with the insurgents for independence, was proclaimed president of the Regency. His Plan of Iguala united factions for independence and envisioned a sovereign nation, with the stated hope that new state would be led by a member of the Spanish royal family or a prince from another European royal house.

The Plan of Iguala, also known as the Plan of the Three Guarantees ("Plan Trigarante") or Act of Independence of North America, was a revolutionary proclamation promulgated on 24 February 1821, in the final stage of the Mexican War of Independence from Spain. The Plan stated that Mexico was to become a constitutional monarchy, whose sole official religion would be Roman Catholicism, in which both the Peninsulares (people born in Spain and residing in Mexico) and the Americanos (people born in Mexico, that is, the Americas) would enjoy equal political and social rights. It took its name from the city of Iguala in the modern-day state of Guerrero.

The two main figures behind the Plan were Agustín de Iturbide (who would become Emperor of Mexico) and Vicente Guerrero, revolutionary rebel leader and later President of Mexico. The Army of the Three Guarantees was formed by the unified forces of Iturbide and Guerrero to defend the ideals of the Plan of Iguala. On 24 August 1821, Iturbide and Spanish viceroy Juan O'Donojú signed the Treaty of Córdoba in Córdoba, Veracruz, ratifying the Plan of Iguala, and thus confirming Mexico's independence.

The military history of Mexico encompasses armed conflicts within that nation's territory, dating from before the arrival of Europeans in 1519 to the present era. Mexican military history is replete with small-scale revolts, foreign invasions, civil wars, indigenous uprisings, and coups d'état by disgruntled military leaders. Mexico's colonial-era military was not established until the eighteenth century. After the Spanish conquest of the Aztec Empire in the early sixteenth century, the Spanish crown did not establish on a standing military, but the crown responded to the external threat of a British invasion by creating a standing military for the first time following the Seven Years' War (1756–63). The regular army units and militias had a short history when in the early 19th century, the unstable situation in Spain with the Napoleonic invasion gave rise to an insurgency for independence, propelled by militarily untrained men fighting for the independence of Mexico. The Mexican War of Independence (1810–21) saw royalist and insurgent armies battling to a stalemate in 1820. That stalemate ended with the royalist military officer turned insurgent, Agustín de Iturbide persuading the guerrilla leader of the insurgency, Vicente Guerrero, to join in a unified movement for independence, forming the Army of the Three Guarantees. The royalist military had to decide whether to support newly independent Mexico. With the collapse of the Spanish state and the establishment of first a monarchy under Iturbide and then a republic, the state was a weak institution. The Roman Catholic Church and the military weathered independence better. Military men dominated Mexico's nineteenth-century history, most particularly General Antonio López de Santa Anna, under whom the Mexican military were defeated by Texas insurgents for independence in 1836 and then the U.S. invasion of Mexico (1846–48). With the overthrow of Santa Anna in 1855 and the installation of a government of political liberals, Mexico briefly had civilian heads of state. The Liberal Reforms that were instituted by Benito Juárez sought to curtail the power of the military and the church and wrote a new constitution in 1857 enshrining these principles. Conservatives comprised large landowners, the Catholic Church, and most of the regular army revolted against the Liberals, fighting a civil war. The Conservative military lost on the battlefield. But Conservatives sought another solution, supporting the French intervention in Mexico (1862–65). The Mexican army loyal to the liberal republic were unable to stop the French army's invasion, briefly halting it with a victory at Puebla on 5 May 1862. Mexican Conservatives supported the installation of Maximilian Hapsburg as Emperor of Mexico, propped up by the French and Mexican armies. With the military aid of the U.S. flowing to the republican government in exile of Juárez, the French withdrew its military supporting the monarchy and Maximilian was caught and executed. The Mexican army that emerged in the wake of the French Intervention was young and battle tested, not part of the military tradition dating to the colonial and early independence eras.

Liberal General Porfirio Díaz was part of the new Mexican military, a hero of the Mexican victory over the French on Cinco de Mayo 1862. He revolted against the civilian liberal government in 1876, and remained continuously in the presidency from 1880 to 1911. Over the course of his presidency, Díaz began professionalizing the army that had emerged. By the time he turned 80 years old in 1910, the Mexican military was an aging, largely ineffective fighting force. When revolts broke out in 1910–11 against his regime, a rebel forces scored decisive victories over the Federal Army in the opening chapter of the Mexican Revolution (1910–1920). Díaz resigned in May 1911, but Francisco I. Madero, on whose political behalf rebels rose against Díaz, demobilized the rebel forces and kept the Federal Army in place. "This single decision cost [Madero] the presidency and his life." Army General Victoriano Huerta seized the presidency of Madero in 1913, with Madero murdered in the coup d'état. Civil war broke out in the wake of the coup. Huerta's Federal Army racked up one defeat after another by the revolutionary armies, with Huerta resigning in 1914. The Federal Army ceased to exist. A new generation of fighting men, most of whom with no formal military training but were natural soldiers, now fought against each other in a civil war of the winners. The Constitutionalist Army under the civilian leadership of Venustiano Carranza and the military leadership of General Álvaro Obregón were the victors in 1915. The revolutionary military men were to continue to dominate Mexico's postrevolutionary period, but the military men who became presidents of Mexico brought the military under civilian control, systematically reining in the power of the military and professionalizing the force. The Mexican military has been under civilian government control with no President of Mexico being military generals since 1946. The fact of Mexico's civilian control of the military is in contrast the situation in many other countries in Latin America.

Since declaring independence in 1821, Mexico has adopted a number of constitutions or other documents of basic law with constitutional effects. Not all these can be considered constitutions, and not all of them enjoyed universal application. Those enacted in 1824, 1857, and 1917 are generally considered full-fledged, operational constitutions. The Constitution of 1824 established the framework of a federated republic, following the short-lived monarchy of Agustín de Iturbide (in 1821–22). The Constitution of 1857 was the framework set by Mexican liberals that incorporated particular laws into the constitution. The Constitution of 1917 was drafted by the faction that won the Mexican Revolution, known as the Constitutionalists for their adherence to the Constitution of 1857. It strengthened the anticlerical framework of the 1857 constitution, empowered the state to expropriate private property, and set protections for organized labor. The 1917 Constitution was significantly revised in 1992 under Carlos Salinas de Gortari, eliminating anticlerical restrictions and strengthening private property rights against the State.

From January 1822 to July 1823, the Captaincy General of Guatemala, a former Spanish colony, was controlled by the First Mexican Empire, and briefly, the Supreme Executive Power—the provisional government that succeeded Mexican imperial rule. The captaincy general consisted of the provinces of Chiapas, Costa Rica, El Salvador, Guatemala, Honduras, and Nicaragua—the six southernmost provinces of the Mexican Empire. The incorporation of Central America brought Mexico to the height of its territorial extent.

Only two months after the Act of Independence of Central America was signed in September 1821, Regent of Mexico Agustín de Iturbide, who later became the emperor of Mexico in May 1822, made a formal request to the Consultive Junta of Guatemala City—the Central American government—to accept annexation to the Mexican Empire. His request was accepted on 5 January 1822. Despite the acceptance by the Guatemalan-based government, El Salvador, Costa Rica, and parts of Nicaragua resisted Mexican annexation, and Mexican and allied Guatemalan soldiers were mobilized to subjugate those regions.

At the end of the Mexican War of Independence, the Army of the Three Guarantees (Spanish: Ejército Trigarante or Ejército de las Tres Garantías) was the name given to the army after the unification of the Spanish troops led by Agustín de Iturbide and the Mexican insurgent troops of Vicente Guerrero, consolidating Mexico's independence from Spain. The decree creating this army appeared in the Plan de Iguala, which stated the three guarantees which it was meant to defend were religion, independence and unity. Mexico was to be a Catholic empire, independent from Spain, and united against its enemies.