Lawmakers moved to block the Department of Defense's move to transfer $1 billion for the construction of a physical barrier along the U.S.-Mexico border, the latest showdown in the battle over the border wall that President Trump has vowed to build.

On Monday, the Pentagon authorized the transfer of funds to Army engineers for projects along the border, which include building 57 miles of 18-foot-high fencing, constructing and improving roads, and installing lighting in support of Department of Homeland Security and Customs and Border Patrol. The move was carried out under the president's declaration of a national emergency, the department said in a statement.

As Acting Secretary of Defense Patrick Shanahan was testifying before his committee, Rep. Adam Smith, the Democrat who chairs the House Armed Services Committee, released a letter denying the Pentagon's move to reprogram $1 billion.

“The House Committee on Armed Services has competed its review of the proposed reprogramming request,” Smith's letter read, according to a copy obtained by Military Times. “The committee denies this request.”

In a more detailed statement, Smith called the Department of Defense's decision to reprogram $1 billion without Congressional approval a violation of trust. “DoD is attempting to circumvent Congress and the American people's opposition to using taxpayer money for the construction of an unnecessary wall.”

Smith noted that the military is paying the price, referencing a recent request by the Marine Corps for additional funding to cover unexpected costs, which included hurricane relief and the troop deployments to the US-Mexico border, among other things.

“This needs to stop,” Smith said, adding: “The administration should stop using our service members as a political tool and instead focus on building military capabilities and readiness in areas where we should focus our defense resources. Congress will act to defend its constitutional prerogatives.”

Shanahan, who appeared before the House Armed Services Committee with Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford and Pentagon comptroller David Norquist, argued that “military construction on the border will not come at the expense of our people, our readiness, or our modernization.”

As Smith warned that the Pentagon could lose its funding flexibility, specifically the department's reprogramming privileges, Shanahan said that the risks have been weighed and the Department of Defense understands the potential consequences.

Rep. Joe Courtney, a Connecticut Democrat, called this a “rubicon moment” for Congress and the Pentagon.

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