U.S.-Mexico Was an Open Border Until It Wasn't

How was the border established?
After the U.S. victory in the Mexican-American War in 1848, Mexico was forced to sign over 525,000 square miles of territory, including what is now California, Nevada, and Utah, as well as parts of Arizona, New Mexico, Colorado, and Wyoming. Five years later, the U.S. purchased another 29,000-square-mile strip of land containing southern Arizona and southwestern New Mexico, more or less creating the present-day border. The sparsely populated region was barely policed, and Mexicans and Americans freely crossed back and forth. In border towns like Nogales, Ariz., saloons straddling the border sold Mexican cigars on the Mexican side and American liquor on the U.S. side to avoid customs duties. Illegal immigration wasn't considered a problem, because for most of the 19th century, the U.S. had virtually open borders. Would-be immigrants "didn't need a passport," said Mae Ngai, a historian at Columbia University. "You didn't need a visa."