WASHINGTON -- The Pentagon began flying military helicopters and surveillance planes over the U.S. border with Mexico last month as part of an effort to withdraw all but 300 of the National Guard ground troops who have helped patrol the rugged border since mid-2010.

The 19-month deployment of 1,200 National Guard troops on the southwest border has hurt recruiting efforts and threatened to strain diplomatic relations with Mexico, Brian J. Lepore, a director at the U.S. Government and Accountability Office, told a House homeland security subcommittee hearing Tuesday.

About 12 Blackhawk helicopters and several fixed-wing manned surveillance planes began flying regular patrols over the Rio Grande in Texas for a mission called “Operation River Watch II” in March. The 300 troops will fly the aircraft, or analyze intelligence about smuggling routes in command centers miles from the border.

The Obama administration deployed the National Guard to build access roads for border patrols and to help spot smugglers. The extra manpower was intended to bridge the gap while U.S. Customs and Border Patrol hired an additional 1,200 agents.

In the first year, the National Guard troops helped apprehend 17,887 illegal immigrants and seize 56,342 pounds of marijuana, which was 5.9% of all apprehensions and 2.6% of marijuana seizures during that time, officials said.

National Guard troops could man watchtowers and stare at closed-circuit television screens of the fence line but were prohibited from making arrests, and officials said morale suffered. The National Guard leadership became concerned that the mission, if extended, could hurt recruitment, according to a GAO report titled “Observations on Costs, Benefits, and Challenges of a Department of Defense Role in Helping to Secure the Southwest Land Border.”

Further use of National Guard troops “could create a perception of a militarized U.S. border with Mexico,” State Department officials told the GAO. The Obama administration has worked with Mexico to strengthen civilian law enforcement agencies to combat drug cartels responsible for thousands of killings.

 “We need to have a long-term vision and whole-of-government approach to securing the southwest border that will replace the ad hoc application of resources that has, to date, epitomized our approach to border security,” Rep. Candice Miller (R-Mich.), who chairs the subcommittee on border and maritime security, said in a statement.

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