This was the scene of the first serious fighting, and visitors this Sunday will get a bonus: At 2 o'clock, hundreds of members of the North-South Brigade will re-enact the Battle of First Manassas at Bull Run Regional Park (not to be confused with the almost-adjoining Manassas National Battlefield Park). Explanations of the action will be narrated over loudspeakers during the breaks between the roar of cannon and the rattle of musketry. Manassas was vital because it was a rail junction. A railroad could move an army farther in a day than it could march in a week, and any army campaigning between Washington and Richmond was open to devastating flank attacks unless it controlled Manassas. In the summer of 1861 Mr. Lincoln's army was made up mainly of three-month volunteers whose time was running out, so it was sent south more or less willy-nilly. On July 18 it ran into similarly raw Southern recruits at Manassas; both sides fought well, but after three days the Southerners prevailed. The Union army withdrew in order, but retreat became rout when sightseers' carriages jammed the road back to Washington. After First Bull Run, as the battle is also called, both sides knew there was a war on. Thirteen months later the two armies, much larger and well-trained, met again at Manassas. Some 24,000 men fell as Lee smashed the Yankee force, clearing the way for his first invasion of the North. Suburbanization has eaten away some of the battlefield, but much remains, including the ground where the central actions took place. MANASSAS NATIONAL BATTLEFIELD PARK -- About 30 miles from Washington via I-66 (exit lavishly marked). Open during daylight. BULL RUN REGIONAL PARK -- Take the Centreville (Rt. 234) exit from I-66 and follow the signs. Open during daylight. Free to residents of Arlington, Fairfax and Loudon counties and the cities of Alexandria, Fairfax and Falls Church; others must pay $3.25 per vehicle to enter. Tent and RV campsites; picnic areas, pool, playgroun, ballfields, skeet and trap range, bridle, hiking and nature trails.