YOUR BASIC HISTORY texbook may not mention it, but the last Battle of Manassas has been in full tilt for a good five years. It has been an uphill legislative fight to reserve land around that famous battlefield from commercial encroachment. And now the news from the front is good: general agreement on a bill has been reached by Rep. Herbert E. Harris (D-Va.), who for years has led the charge for preservation, and by Virginia's two senators, whose support is crucial to congressional approval of any such measure.

This battle is important not just to Manassas or the state of Virginia, but to the country; for without enactment of a bill, more than 1,000 acres of historically valuable land surrounding Manassas Park remains under threat of permanent ruination by commercial developments. Local officials, including a majority of the Prince William County Board of Supervisors, have been more than a little eager over the years to ring the battlefield park with "gettysburg-type development" -- even though there are thousands of industrial and commercial acres available elsewhere in that area.

The outlook for the legislation improved greatly last year, after William L. Scott, who had repeatedly blocked bills, was replaced in the Senate by John W. Warner, who joined the effort to reach an agreement. In addition, Sen. Harry F. Byrd Jr. ended years of resistance, and Mr. Harris worked to incorporate suggestions made by various local property owners and officials and managed to strengthen support for a congressional measure.

Though there are still some differences between House and Senate proposals, all three Virginia legislators believe a compromise bill can be enacted this year. Speed is essential, for Prince William is one of the fastest growing countries in the country, and pressures for development are mounting.