The Border Patrol, however, concerns itself far less with counterterrorism than with the agency’s traditional tasks of immigration and drug enforcement. This creates an uneasy mixture of missions. And it results in the deployment of an expensive military apparatus to police and capture immigrants who cross the border in the hopes of finding jobs as maids, janitors or day laborers.

In 2012, a majority of the more than 364,000 people arrested by Border Patrol agents nationwide were migrant workers crossing the border. Agents did not capture or arrest a single international terrorist.

But they have disrupted the lives of tens of thousands of people like Stewart Loew who live and work near the border. There’s a point on Interstate 19, two miles from the Loews’ farm, that buzzes with what borderland residents call the “men in green,” who stop and interrogate everyone who drives past. Border Patrol vehicles scan the off-road areas smugglers and migrants use to circumvent official checkpoints. A mobile control tower with a sophisticated surveillance system mounted on its cabin is visible near the Loews’ farm.

The Department of Homeland Security, which includes Customs and Border Protection, plans to invest billions more in borderland surveillance towers, drones and helicopters if the House adopts the immigration reform bill that the Senate passed in June. Even if it doesn’t pass, there is more than $1 billion in the federal budget for surveillance towers that will likely be clustered around the Arizona desert lands, near Mr. Loew’s farm, where most undocumented migrants cross the border.

The Republican senators Bob Corker of Tennessee and John Hoeven of North Dakota added a proposal to the immigration bill that would provide about $40 billion in financing for extra agents and 700 miles of fencing along the United States’ southern boundary, which Senator John McCain, Republican of Arizona, noted would become “the most militarized border since the fall of the Berlin Wall.” Indeed, if the Senate’s bill, the Border Security, Economic Opportunity and Immigration Modernization Act, passes in the House, the Border Patrol will swell to 40,000 agents, making it the size of a small army.

In recent years, we have built up our boundary and immigration policing apparatus with great speed. Founded in 1924, the Border Patrol deployed just over 4,000 agents in 1993. In only 20 years the agency’s ranks have more than quintupled, and if the reform passes it will increase its size tenfold.