The Battle of the Alamo (February 23 – March 6, 1836) was a pivotal event and military engagement in the Texas Revolution. Following a 13-day siege, Mexican troops under President General Antonio López de Santa Anna reclaimed the Alamo Mission near San Antonio de Béxar (modern-day San Antonio, Texas, United States). About one hundred Texians, wanting to defy Mexican law and maintain the institution of chattel slavery in their portion of Coahuila y Tejas by seeking secession from Mexico, were garrisoned at the mission at the time, with around a hundred subsequent reinforcements led by eventual Alamo co-commanders James Bowie and William B. Travis. On February 23, approximately 1,500 Mexicans marched into San Antonio de Béxar as the first step in a campaign to retake Texas. In the early morning hours of March 6, the Mexican Army advanced on the Alamo. After repelling two attacks, the Texians were unable to fend off a third attack. As Mexican soldiers scaled the walls, most of the Texian fighters withdrew into interior buildings. Those who were unable to reach these points were slain by the Mexican cavalry as they attempted to escape. Almost all of the Texian inhabitants were killed.

Several noncombatants were sent to Gonzales to spread word of the Texian defeat. The news sparked both a strong rush to join the Texian army and a panic, known as "The Runaway Scrape", in which the Texian army, most settlers, and the government of the new, self-proclaimed but officially unrecognized Republic of Texas fled eastward toward the U.S. ahead of the advancing Mexican Army. Santa Anna's execution of surrendering soldiers during the battle inspired many Texians and Tejanos to join the Texian Army. The Texians defeated the Mexican Army at the Battle of San Jacinto, on April 21, 1836, ending the conquering of the Mexican state of Coahuila y Tejas by the newly formed Republic of Texas.

Within Mexico, the battle has often been overshadowed by events from the Mexican–American War of 1846–1848. In 19th-century Texas, the Alamo complex gradually became known as a battle site rather than a former mission. The Texas Legislature purchased the land and buildings in the early part of the 20th century and designated the Alamo chapel as an official Texas State Shrine.

The Alamo has been the subject of numerous non-fiction works beginning in 1843. Most Americans, however, are more familiar with the myths and legends spread by many of the movie and television adaptations, including the 1950s Disney miniseries Davy Crockett and John Wayne's 1960 film The Alamo.

In 1835, following the repeal of the federalist 1824 Constitution of Mexico, Mexico adopted Las Siete Leyes (“The Seven Laws”), a new constitutional framework that replaced the First Mexican Republic with a unitary republic officially named the Mexican Republic (Spanish: República Mexicana). The laws were formalized on 15 December 1835 under President Antonio López de Santa Anna and enacted in 1836. Intended to centralize and strengthen the national government, they reflected conservative leaders’ belief—after a decade of political instability, economic stagnation, and foreign threats—that Mexico required a more centralized model than the U.S.-inspired federalism of 1824.^{[citation needed]}

New policies such as banning slavery, changes to immigration, and increased enforcement of laws and import tariffs incited many immigrants to revolt. The border region of Mexican Texas was largely populated by immigrants from the United States, some legal but most illegal. Some of these immigrants brought large numbers of slaves with them, so that by 1836, there were approximately 5,000 enslaved persons in a total non-native population estimated at 38,470. These people were accustomed to a federalist government which made special exemptions from Mexican law exclusively for them, and to extensive individual rights including the right to own slaves. This led to their growing displeasure at Mexico's law enforcement and shift towards centralism when the centralized government ended local federal exemptions to the ban on slavery, previously negotiated by Stephen Austin and others. Already suspicious after previous United States attempts to purchase Mexican Texas, Mexican authorities blamed much of the Texian unrest on United States immigrants, most of whom had entered illegally and made little effort to adapt to the Mexican culture, and who continued the practice of slavery after it had been abolished in Mexico.

In October, Texians engaged Mexican troops in the first official battle of the Texas Revolution. Determined to quell the rebellion of immigrants, Santa Anna began assembling a large force, the Army of Operations in Texas, to restore order. Most of his soldiers were less-experienced recruits, many conscripted.

The Texians systematically defeated the Mexican troops already stationed in Texas. The last group of Mexican soldiers in the region—commanded by Santa Anna's brother-in-law, General Martín Perfecto de Cos—surrendered on December 9 following the siege of Béxar. By this point, the Texian Army was dominated by very recent arrivals to the region, primarily illegal immigrants from the United States. Many Texas settlers returned home, unprepared for a long campaign. Angered by what he perceived to be United States interference in Mexican affairs, Santa Anna spearheaded a resolution classifying foreign immigrants found fighting in Texas as pirates. The resolution effectively banned the taking of prisoners of war; in this period of time, captured pirates were executed immediately. Santa Anna reiterated this message in a strongly worded letter to United States President Andrew Jackson. This letter was not widely distributed, making it likely that most of the United States recruits serving in the Texian Army were not aware there would be no prisoners of war.