More than 70,000 people cross legally every single day at San Ysidro, the busiest land border crossing in the world, and you have reached the edge of one of the most consequential lines on Earth. The border between the United States and Mexico was established by the Treaty of Guadalupe Hidalgo in 1848, which ended the Mexican-American War and transferred half of Mexico's territory to the United States, drawing a line through communities, families, and landscapes that had been continuous for centuries. Before the border, this was all the same place.
Tijuana, directly across the line, began as a small Mexican customs post and grew into a city of two million people — one of the largest and most economically dynamic cities on the US-Mexico border. During Prohibition from 1920 to 1933, Tijuana and its bars, casinos, and racetracks became a release valve for San Diego's and Los Angeles's demand for alcohol and entertainment, and rum-running networks moved bootleg liquor north through this very crossing. Today the border zone is defined by the infrastructure of enforcement — fencing, cameras, lights, vehicle barriers — and by the equally powerful infrastructure of daily life that crosses in both directions: workers, students, shoppers, families, and the trucks carrying billions of dollars in cross-border trade.
What you see here is America's most contested seam.