America and Mexico: Texas, the War, and the Border
The 2,000-mile line between the United States and Mexico is one of the most consequential borders on earth, and almost none of it was settled peacefully. In a single generation the United States annexed an independent Texas, fought a war that took the capital of Mexico City, and walked away with the land that became California, Arizona, New Mexico, and more — roughly half of Mexico's territory. Everything in the relationship since has been shaped by that founding asymmetry.
This guide follows the relationship from the ideology that drove American expansion, through the Texas revolt and the war it led to, to the treaty that fixed the border and the trade that now binds the two economies. Each entry links to a full account; together they explain how two nations became permanent, deeply entangled neighbors.
The road to the border began with an idea: that the United States was destined to stretch from the Atlantic to the Pacific, whatever stood in the way. What stood in the way was Mexico, which had won its own independence from Spain only in 1821 and held the vast northern territory Americans coveted. The friction concentrated in Texas, where American settlers invited into Mexican land soon revolted, declared a republic, and then asked to join the United States — a request that made war with Mexico all but certain.
Annexation lit the fuse; a border dispute fired the first shot. The war that followed was brief, lopsided, and enormous in its consequences — American armies marched all the way to Mexico City, and the peace that ended the fighting transferred roughly half of Mexico's territory to the United States. The entries here cover the president who willed the war, the campaign itself, and the treaty that turned conquest into a permanent border.
The war set the border, but the decades after it kept testing the line. A final purchase rounded out the map in the 1850s, and during the upheavals of the Mexican Revolution and the First World War the border became a theater of intrigue — most dramatically when Germany secretly offered to help Mexico win back the very territory it had lost. These entries cover that restless half-century between the war and the modern relationship.
If the nineteenth century set the border by force, the twentieth and twenty-first have been defined by everything that crosses it — goods, capital, and people. The relationship today is less about territory than about integration: deeply linked economies, vast trade, and the perennial politics of migration. This final entry marks that shift from conquest to interdependence.
The war with Mexico was one chapter in a continental story of expansion — see every U.S. war and major conflict — and the rivalry it left behind echoes the pattern traced in the other relations guides.