They called it the "Third Battle of Manassas" and it lasted longer than the entire Civil War. But when the dust finally settled on the bitter dispute over expanding the boundaries of Prince William County's historic battlefields, Congress had approved it, Jimmy Carter had signed it and the money to buy the land appeared to be on its way.

So much for appearances.

This week authorities at Manassas National Battlefield Park received notice that all plans for implementing the expansion have been placed on an indefinite hold. As a result of a moratorium announced in February by Interior Secretary James G. Watt, the Reagan administration plans to spend no money on buying new parkland this year, including 1,500 acres designated by Congress for the Manassas battlefield.

"I guess you might say we're in a holding pattern," said Bill Kling, press spokesman for Sen. John W. Warner (R-va.), who was instrumental in engineering the compromise that pushed the Manassas expansion through Congress last year, supposedly ending a six-year fight over the issue. "We're not back to sqaure one," said Kling. "We've come a long way.But budget priorities are standing in the way right now."

The latest announcement has thrown into doubt the future of lands Warner's legislation placed inside the park's expanded boundaries. Park authorities said the hold has the effect of placing a cloud over the title to lands of about 60 individuals. They can sell their land to the first bidder, but cannot assure prospective buyers that the federal government won't step in later and want the property for the park.

"I really feel for some of these people who were expecting to sell," said battlefield park superintendent Rolland Swain, who notified landowners of the administration's plans in a May 8 letter. "Now they're kind of in between and don't know exactly what to do."

Swain had planned to begin purchase of the park's new acreage in October with $2 million of the $8.7 million granted in the legislation.

The land, which includes three key historic sites -- one of them nominated for the National Register of Historic Places -- is located less than three miles from the intersection of I-66 and Rte. 234 about 30 miles west of Washington. The area is rapidly developing, raising concerns that even if the area is not sold to commercial builders, it may become too expensive for the government to acquire.

"Don't let anybody kid themselves that they'll acquire it in the future because it will be gone," said Herbert E. Harris, Virginia's former 8th District Democratic representative who introduced legislation to expand the park repeatedly, only to see it die in the Senate.

Harris denounced Watt's action. "It represents the destruction of five years of effort to preserve land as precious as any we have on the East Coast. It really undercuts the work of people clear across the country."

Harris's early efforts to fund the acquisition were blocked in the Senate by Virginia's Independent Harry F. Byrd Jr., and Warner's predecessor, Republican William L. Scott. Both declined to override objections by members of the Prince William County Board of Supervisors, who were determined to see part of the area around the park set aside for commercial development.

Congress has approved $233.6 million for acquisition of park lands around the country this year. Under Watt's moratorium, the budget has been paired to $29 million, which would be held as a contingency against hardship cases and land disputes still in the courts.