WASHINGTON – Military construction and family housing accounts would pay for the troops and materials needed to build barriers on the southern border should President Trump declare a national emergency there.

First, however, he'll have to convince Pentagon lawyers that the situation there constitutes a threat to national security – a true national emergency, and not a law enforcement matter, said Todd Harrison, a military budget expert at the Center for Strategic and International Studies, a non-partisan think tank.

"If we had intelligence that an army was massing on our southern border that would clearly be a national emergency," Harrison said. "That’s clearly not the case here."

A 19th century law, the Posse Comitatus Act, prohibits the use of federal troops for law enforcement under most circumstances.

"There needs to be a true military purpose for the emergency, not just law enforcement," Harrison said.

After the Sept. 11 terror attacks, President Bush declared a national emergency, a move that allowed money to be re-directed to build military facilities in the Middle East. Trump, in a prime-time address Tuesday night, did not declare a national emergency. Instead, he described dire consequences from illegal immigration and called on Congress to fund a barrier and other measures to secure the border and provide humanitarian relief to immigrants.

Rep. Adam Smith, the Washington Democrat and new chairman of the House Armed Services Committee, said in a statement that the declaration of a national emergency at the border would be an "unwise, weak and irresponsible legal gimmick" to pay for the wall.

"President Trump is considering a plan to misuse an emergency authority and divert substantial resources from our military to build a wall on the southern border," Smith said. "This is as clear a statement as any that President Trump values the construction of a wall over military readiness and support for our troops and their families."

Assuming the White House can convince Pentagon lawyers of the emergency, then money earmarked but not spent on contracts for projects like landing strips, military hospitals and troops' barracks could be used to pay for the wall, Harrison said.

The Trump administration in the fall sent thousands of active duty troops to the southern border to support border patrol agents. The troops strung concertina wire at border crossings and provided transportation at the request of the Department of Homeland Security.

There are about 600 active duty troops in Texas, 650 in Arizona, and 1,100 in California, according to Army Lt. Col. Jamie Davis, a Pentagon spokesman. Their deployment is set to end Jan. 31. However, last week Homeland Security officials requested further assistance from the Pentagon in building barriers and for aircraft.

An additional 2,200 National Guard troops are at the border with deployments scheduled through September.

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