For the past two decades the United States, a country with a strong tradition of limited government, has been pursuing a widely popular initiative that requires one of the most ambitious expansions of government power in modern history: securing the nation's borders against illegal immigration. Congress and successive administrations—both Democratic and Republican—have increased the size of the Border Patrol from fewer than 3,000 agents to more than 21,000, built nearly 700 miles of fencing along the southern border with Mexico, and deployed pilotless drones, sensor cameras, and other expensive technologies aimed at preventing illegal crossings at the land borders. The government has overhauled the visa system to require interviews for all new visa applicants and instituted extensive background checks for many of those wishing to come to the United States to study, travel, visit family, or do business. It now requires secure documents—a passport or the equivalent—for all travel to and from the United States by citizens and noncitizens. And border officers take fingerprints and run other screening measures on all travelers coming to this country by air in order to identify criminals, terrorists, or others deemed to pose a threat to the United States.