La Segunda Compañía Volante de San Carlos de Parras (Álamo de Parras), a company of 100 Spanish lancers, arrived in Texas in January 1803. The men came from the pueblo of San José y Santiago del Álamo, near Parras in southern Coahuila. Under the jurisdiction of the curacy of the Villa de San Fernando and the Bishop of Nuevo León, they occupied the secularized mission of San Antonio de Valero. Indian raids, cattle rustling, and smuggling plagued the Texas province. Spain's return of the Louisiana territory to France in 1800 gave rise to a threat of an American invasion. San Antonio de Béxar's only protection came from a small group of soldiers garrisoned at the local presidio. The detachment of the Álamo de Parras company was one of several reinforcement efforts. As seasoned veterans the men provided increased protection from Indians, thereby significantly reducing the theft of livestock and the smuggling of other goods. As a mounted contingent they served at times as couriers and escorts to the Spanish governor. In place of the company's long name a shortened one, La Compañía del Álamo, or simply El Álamo, became commonly used. Through this association Mission Valero came to be known as the Alamo.
There were more than 200 men, women, and children in the Alamo military community within the mission's walls. Some occupied the abandoned quarters inside the walls of the compound. Other soldiers with families lived in jacals, crude huts initially built within the compound for protection. Unmarried soldiers were housed in the lower floor of the mission's convent, called the Long Barracks, or in buildings procured nearby. The Álamo de Parras Company was subsequently joined by the companies of Nuevo León and Nuevo Santander, further adding to the acute need for additional housing. As intermarriage occurred within the community, a settlement grew near the mission known as El Barrio del Álamo. Later marriages and a subsequent surge in population were the basis for a district near the mission on the banks of the San Antonio River called La Villita. The Álamo de Parras Company founded the first military hospital in San Antonio by 1805, in the upper level of the mission convent. It served both the military and civilian populations.
In 1809 rumors of an invasion by the United States spread across the province. The Álamo de Parras Company promptly ordered materials to add 834 varas of battlement to the existing walls of the enclosure. Part of this construction included the southernmost wall with a large gate separating a new barrack from a guardhouse and jail. During the Casas revolt of 1811, the opposing junta enlisted officers of the Álamo de Parras Company in their counterrevolt. The officers arrested Governor Juan Bautista de las Casas and his advisors and were recognized for their part in ending the rebellion. In 1813 forces of the Gutiérrez-Magee expedition occupied San Antonio. Facing a superior force and possible imprisonment, royalist troops from the Alamo and San Antonio de Béxar Presidio surrendered without resistance. Many quickly joined the Americans. The commandant of the Álamo de Parras Company, Lt. Vizente Tarín, left his command to become a captain in the insurgent army. Private Pedro Prado was implicated in the assassination of Governor Manuel Salcedo and Lt. Col. Simón Herrera. Both Tarín and Prado participated in the battle of Medina. When Gen. Joaquín de Arredondo entered Texas on his mission of retribution, those unresolved renewed their Spanish loyalties. Others, determined to fight, fled for the safety of the Neutral Ground and Louisiana to continue their insurgent activities.
The Alamo fort was virtually abandoned. The soldiery dwindled to an ineffective constabulary, leaving the city open to Indians and rebels who raided the villa with virtual impunity. Continuing revolt and a depleted treasury hampered Governor Antonio Martínez's efforts to restore the military. In the fall of 1817 the order came to fill the ranks of the Álamo de Parras Company. Detached from another province, seventy-five men from the veteran company of Nuestra Señora del Carmen left for the Valero fort and San Antonio de Béxar. Local conscripts filled the remaining vacancies in the company.
After the Mexican War of Independence, Gen. Manuel de Mier y Terán set military garrisons on the frontier to promote Mexican expansion and check illegal immigration and smuggling. In 1830 the Álamo de Parras Company built and occupied Fort Tenoxtitlán under the command of Lt. Col. José Francisco Ruiz. Civil war in the Mexican capital, as well as a lack of interest in Terán's colonization plan, hastened the decline of the garrison system. Living conditions at Tenoxtitlán grew worse, and desertions were commonplace as support from Mexico was withdrawn. Eventually, the troops were recalled to help meet the crisis in the capital.
The Álamo de Parras Company abandoned Fort Tenoxtitlán on August 22, 1832. They arrived with their families in San Antonio de Béxar on September 14. Most of El Álamo's soldiers joined troops leaving from Nacogdoches on a march to Matamoros. A few remained behind, but the political climate and continued neglect caused some to leave the company, either by resignation or desertion. Some defected to the revolutionary army of Texas. Privates Pedro Herrera, Nepomuceno Navarro, and Manuel Tarín served in Col. Juan N. Seguín's company of Tejanos throughout the revolution and fought at San Jacinto. Private José Toribio Losoya, also under Seguín, was killed at the Alamo during its final hour; his body was burned with those of the other defenders.
In December 1835 a remnant of Álamo de Parras troops occupied the Alamo and joined with the army regulars of Gen. Martín Perfecto de Cos to defend San Antonio during the siege of Bexar. With the defeat of his army, Cos surrendered San Antonio and the Alamo on December 11, 1835. The Texans granted the Mexican army permission to return to Monclova. This ended the Álamo de Parras Company's association with the Valero mission.