The Army has been stopped by an old red brick wall. And some federal planners -- at least temporarily.

The National Capital Planning Commission, which must approve all federal building plans in the Washington area, last week blocked the Army from building a $25 million education complex next to Fort McNair in Southwest Washington.

The Army wanted to tear down 400 feet of the brick wall that forms the perimeter of the fort and separates the proposed site for the new building from the rest of the fort.

But the planning commission said removal of a portion of the wall and construction of the large three-story building that would include a library, computer school and offices would adversely affect the balance achieved in the fort's "magnificent" and historic master plan.

The Army's plan "detracts significantly from the setting of the War College Building and is inconsistent with the basic design concepts of the 1903 master plan for this historically important post," according to the commission.

The planners want a smaller building at the proposed location and indicated they might accept a small opening in the wall. The Army has gone back to the drawing boards.

"We are responding to the comments, and we are confident we will be able to satisfy the requirements of the commission ," said Capt. Steve Hatch, military construction representative for the National Defense University.

"Our architect will do whatever is necessary to meet their requirements, and I don't think that we're far off, frankly," he added.

The 87-acre Fort McNair, the oldest U.S. fort in continuous use in the country and home of the military district of Washington, begins at Fourth and P streets SW, next to a residential neighborhood.

It resembles a college campus more than a military post. Under the 1903 plan, an expansive grassy parade ground runs 3,000 feet from a grouping of columned buildings just inside the gates to the giant dome-roofed War College on the Anacostia River.

Flanking the parade ground on the west are a series of large columned mansions facing the river and Hains Point that are the prized residences of colonels and generals.

The planning commission and Commission of Fine Arts, a federal agency that is the arbiter of esthetics in the federal domains of the city, both objected to the new building, saying the 1903 plan grouped major buildings on a north-south axis and the library's planned location to the east would unbalance that plan.

"Putting such a large building off the center parade ground really disrupts the master plan, which is really what the commission wants to see preserved," said Charles Atherton, secretary of the fine arts commission. "We see no reason to depart from the plan when it is possible to accommodate the building into the existing plan for Fort McNair ."

The library, formally known as the National Defense University Academic/Library Center, would be built on 10 acres of land replacing two large temporary buildings built during World War II. The buildings are now vacant. The plan would include within the site a 250-car parking lot.

The university, which offers 10 months of advanced instruction in military strategy to officers and civilian federal employes, has grown from 590 students, faculty and staff in 1975 to 930 last year, said Col. Michael Alba, the project manager.

He attributed the increase to efforts by the Joint Chiefs of Staff and the Department of Defense to promote more advanced instruction in military strategy and national security for military officers and government officials.

The library plan must also be approved by the Armed Services committees and the military construction subcommittees of the House and Senate.

Fort McNair was first designated as a military post in 1791 by Pierre L'Enfant, who planned Washington, according to planning commission reports. It was established for the defense of the federal city. It was the site of the U.S. Penitentiary where four of the Lincoln assassination conspirators were tried and executed July 7, 1865.