The top Democrat on the House Armed Services Committee on Tuesday denied the Pentagon's request to transfer $1 billion for barrier construction on the southern border.

The move follows a notification from the Pentagon on Monday night that the department intended to shift funding to construct 57 miles of 18-foot-high fencing.

While largely symbolic, this move by lawmakers could be the start of a battle between the Pentagon and Congress over spending authority.

Lawmakers blocked the Department of Defense's move to transfer $1 billion for the construction of a physical barrier along the US-Mexico border, the latest showdown in the battle over the border wall that President Donald 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 the Department of Homeland Security and Customs and Border Protection. The move was carried out under the president's declaration of a national emergency, the department said in a statement.

Read more: Defense Department redirects $1 billion to fund US-Mexico border projects

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

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

The Pentagon has long been allowed to reprogram funds to address evolving departmental needs, but pushing ahead on the president's controversial wall without congressional approval was apparently a step too far.

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 said 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.

Read more: The Marine Corps' top general says Trump's deployments to US-Mexico border are hurting combat readiness

"This needs to stop," Smith said, adding: "The administration should stop using our servicemembers 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."

Read more: Trump's border wall may strip money from a $65 million water treatment plant at a Marine Corps base with a troubled history of contaminated water

Shanahan — who appeared before the House Armed Services Committee with Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, and David Norquist, the Pentagon's comptroller — said that "military construction on the border will not come at the expense of our people, our readiness, or our modernization."

The Department of Defense can technically still move forward with its plans to transfer $1 billion for border-security projects, but there could be consequences.

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 significant downside of losing what amounts to a privilege."

Rep. Joe Courtney, a Connecticut Democrat, called this situation a "Rubicon moment" for Congress and the Pentagon.