Disgruntled with the Mexican centralist government's rule by dictatorship and its complacency in defending the northern frontier from Indian attack, many Laredoans supported the constitutional convention which created the Republic of the Río Grande on January 7, 1840. Laredo became a capital of the new republic which attempted to unite Tamaulipas, Nuevo Leon, and parts of South Texas. After several skirmishes with the Mexican army, the short-lived republic came to an end, enduring only 283 days. Although the Republic of Texas, which had won its independence from Mexico in 1836, attempted to claim Laredo, its citizens remained loyal to Mexico after the defeat of the Republic of the Río Grande.
In 1845, the annexation of Texas by the United States led to the declaration of war against Mexico. Shortly after the fall of Mexico, the Río Grande was declared the boundary between the United States and Mexico. Under the Treaty of Guadalupe Hidalgo, Laredo officially became part of Texas. Mexicans who wanted to retain their citizenship moved across the river.
-Handbook of Texas Online: Webb County; Laredo Convention and visitor's Bureau: History of Laredo