Stretching nearly 2,000 miles from the Pacific Coast to the Gulf of Mexico, the Mexico-U.S. border is the world’s single most crossed international boundary. With over a billion dollars worth of goods moving between the countries every day and 11 million people living within the border region, the history of the Mexico-U.S. boundary line is worth exploring.
There are hundreds of years of history of European powers jockeying for position in North America,
but we’re going to skip ahead to the 19th century with the Adams-Onís Treaty. This 1819 agreement between U.S. Secretary of State John Quincy Adams and Spanish Envoy Lord Don Luis de Onís defined the boundary of American and Spanish territories in North America. It established the northern borders of (modern day) California, Nevada, and Utah, followed the Arkansas and Red Rivers, and formed the Eastern edge of Texas, which is the Sabine River.
Mexico declared independence from Spain in 1821 and soonafter became increasingly concerned with the number of American settlers arriving in Texas during the 1820s. Their growing unease about American incursion into their territory came to fruition when U.S. President Andrew Jackson attempted to buy Texas from Mexico in 1829 for five million dollars. The Mexican government flatly refused. Texas, on the other hand, decided it’d rather not be apart of either country and declared their own independence in 1836.
Part of Texas’ creation of The Republic of Texas was establishing new borders with its neighbors. Texas used the Adams-Onís as a boundary line to the U.S., but decided to set the Rio Grande (called the Río Bravo del Norte in Mexico) as its boundary line to Mexico. Mexico, who refused to formally recognize Texas’ independence in the first place, pushed back against Texas’ desire to set their Republic’s border at the Río Grande. Mexico contested that Texas had to move the line back to the Nueces River, which runs through modern day Corpus Christi.
The dispute surrounding assigning the border at the Río Grande or at Nueces River, coupled with the U.S. annexation of Texas in 1845, set the Mexican-American War into motion. This slice of land between the Río Grande and the Nueces River is called the Trans-Nueces, which you can see in the middle of the two yellow lines in the center of the map on the left. Lasting from 1846-1848, the Mexican-American War ended in the Mexico-U.S. border being set at the Rio Grande and the signing of the Treaty of Guadalupe Hidalgo. As part of the Treaty, Mexico lost a devastating 55% of its land to the U.S., giving both countries the border we recognize today. Well, for the most part.
The problem with setting your border along a natural feature is that natural features can move! The Rio Grande has changed course numerous times, leading to more disputes between both the U.S. and Mexico, and between individual U.S. states. Flooding, falling banks, and land loss were prevalent issues in the decades following the Treaty of Guadalupe Hidalgo and led to complaints from land owners on either side of the Rio Grande. To address the movement, the International Boundary and Water Commission (IBWC) was established to administer treaties and monitor the living border between the U.S. and Mexico. You can get daily updates from the IBWC about flow conditions in the Rio Grande and other related environmental reports about the border that both Mexico and the U.S. care for, together.