WITH ALLIED FORCES IN THE FIELD - The battered iron bridge was the key to the position. Unless the Allies held it the Germans could at one stroke cut off the Free French outpost and drive a wedge between the British and American forces on the north bank of he rain-swollen river.

The initial assault was turned back, but the machine gun mounted in Capt. Mike Williams' jeep jammed as the gray-clad horde came on again, automatic weapons chattering and potato-masher grenades arcing through the smoke. Soon all was silent. The clouds drifted away to reveal Tommies and GIs scattered like rag dolls at the feet of the triumphant Wehrmacht troops.

Lt. George Peterson, 42, of Springfield, Va., lay face-down beside the jeep, fingers still clutching his empty Thompson submachinegun. The body of Captain Williams, 44, his longtime buddy from Fredericksburg, was draped over the gun mount. Wisps of smoke curled from his field jacket.

Peterson's eyes fluttreed.

His lips moved. A combat correspondent knelt in the mud to catch the last words. "Mike," came a whisper and then, more strongly, "Mike." He groaned. "Mike, I tell you, the trouble with this outfit is that there are too damn many Germans. It isn't any fun when the Heinies win all the time."

"Yeah," Williams said, looking underneath to make sure the smoke grenade hadn't set fire to his beloved 1942 Ford jeep. "But what can you do? The Krauts have the classy uniforms and equipment. You can't get the guys away from that stuff."

The outfit Peterson and Williams - and the "Germans" - belong to is the World War II Historical Reenactment Society, Inc., whose members make the weekend woods ring as they play at the war their fathers fought. The members wear government-surplus uniforms and use authentic weapons, including tanks and airplanes. The Second World War may have been hell, but the Second Second World War is a hell of a lot of fun.

"This is a total hobby, collecting the equipment and using it," Williams said. "I run a construction company all week and this clears my mind. It takes a lot of reseach to learn what gear is authentic to a given unit in a given battle, and that leads into the rest of the history of the war. And it gives me and a lot of other guys an opportunity to play out our fantasies."

It gives their wives an opportunity to spend a lot of weekends alone, Williams conceded, "but most of them understand that boys will be boys. A lot of them have gotten into it too. They serve as nurses and ambulance drivers and so forth."

Most of the weekend warriors are reluctant to tell how much they paid for their outfits, lest the word get back home. "You can easy drop a thousand on a good German machine gun," one dealer said. Williams' Jeep, a rare model, cost him "a hell of a lot more than it cost the Army," he said. "And then when I pull the trigger on that .30 caliber, it's like shoveling dimes down a rathole. Still, you can make a case for treating these things as an investment. There are just so many of them around, and the demand keeps growing all the time. I could turn a sizable profit if I sold out."

Williams is Commander of Allied Forces in the local chapter of the society, but wears only a captain's bars because when all his troops are mustered they could fit in a French freight car. At the recent exercise on his brother's farm in Gettysburg, after the Williamses had billeted the Free French, the British 21st Airborne and the 29th U.S. Infantry, there was room to spare in the old Pennyslvania Dutch barn for the Gross Deutschland Division, the 1,th Pioniers and elements of the Leibstandarte Adolf Hitler.

"Actually we have quite a few more members, but they tend to be sunshine soliders," he said. "Comes a little rain, they stay home and go bowling." Watching a late arrival's car sink axle-deep in the mud, he added, "Can't say that I blame them. This is a little too authentic to suit me."

The twoscore members who reported for duty that weekend were dedicated indeed. They fought in rain, they fell in mud, they ran up the hill and down again. At meal-times they huddled in the drafty barn and ate C rations. All the while they were wearing uniforms and using weapons and equipment that would fetch astonishing prices at a military collectors' trade show.

"The quality of this stuff is fantastic," said Peterson, who acts as quartermaster between battles and could outfit a regiment - friendly or enemy - from his basement. "Some of it has been through two wars. Uncle Sam didn't cut any corners when he sent his boys off to the front, and much of the German gear is even better."

With their equipment given such hard use, asked the rookie war correspondent, won't the society soon find itself threadbare? "Not in my lifetime," Petrson said. "You'd be amazed at how much Allied material can still be found in warehouses in the Far East. I make a couple of buying trips a year, and sometimes you even find unopened crates."

Some members will accept Korean War - vintage items - the early part of that conflict was fought with essentially the same equipment - but the purists check lot and serial numbers and won't buy anything made after 1944; the ultimate is gear that crossed the bloody sands of Normandy and the blasted bridges of the Rhine.

There is little interest in memorabilia from Korea, which was a grubby little war in a faraway place, or from Vietnam, which hasn't cooled off yet and anyway had few of the set-piece battles that game well. (There are the further problems of finding Viet Cong and deciding who are the good guys and the bad guys.) Equipment from World War I is too scare and too fragile. The Civil War also has a lot of replay fans, but their reenactments are necessarily done largely with reproductions, which the World War II group regards as heresy.

Very few of the members have ever been in the service and fewer still in a war zone. "Maybe if we had been there it wouldn't be so much fun," Williams said. "We do our best to be authentic, but we don't have enough experienced members, and besides, it's hard to get some of these clowns to drill. They want to dress up and get straight to the boom-boom part."

The correspondent,who had had the good fortune to be drafted after Korea and before Vietnam, assured him that the fun quotient of basic training is negligible.

By far the best-trained unit at the exercise was the British, who not only turned to with precision but even spoke passable "Brit." Their commander, a 4-F because of rheumatic fever, said he preferred the Tommies because "They were rather smarter units than the GIs, a professional army like the Wehrmacht, but there is no language problem. I have always been sorry to have missed the military experience - it's part of what went with being a man when I was growing up - and this gives me a chance to get at least a taste of it."

The society has chapters nationwide, and the big event of the year is when all the troops gather near St. Louis to do D-Day. Most of the weekend outings involve small tactial exercises, as at Gettysburg, although there are frequent larger maneuvers, called "gigs," such as one to be held this weekend at Fort Meade. Army posts and National Guard units are quite supportive of the society, Williams said, "because we honor the military tradition and our public demonstrations help them in recruiting." Membership inquiries should be directed to the World War II Historical Reenactment Society, Inc., P.O.Box 805, Springfield, Virginia 22150.

Society members are required to present an authentic "impression," which means they not only have to have the right equipment but must sacrific the long hair and beards that are currently the fashion among both hippies and hardhats. Those who can't bear baring their chins and ear gravitate toward irregular units like the Free French, whose members tend to go off in the woods to light firecrackers and drink wine.

The society publishes a newsletter full of technical data and accounts of mock battles in which the winners and losers dispute the facts as vehemently as, say, General of the Army Omar Bradley and Field Marshal Montgomery. The endless fascination with equipment was perhaps the least authentic element of the impression the society members left with this correspondent. Whereas Topic A among the soldiers he served with was always women, these guys talk about army stuff.

Recruiting is done mainly through posters at gun shops, surplus stores and at military memorabilia shows such as the ones held each spring at the Greenbelt Armory and in Baltimore. Williams said the nut element is a surprisingly small probelm. "I thought we would draw quite a few weirdos, but it turned out that the guys who come to us are almost all serious collectors who just want a chance to use their gear.

"We don't have any neo-Nazis that I know of, and none need apply. There is one couple that shows up at some of the outings. They have this prewar Mercedes sedan and wear full Nazi regalia, especially her - she's leather from head to toe, and carries this little whip. But I don't think their thing is military, you know?

The society focuses strictly on the European theater (not including Russian equipment, even though Russia's forces were comparable to those of the other Allies and the Germans together). The Pacific War is pretty much ignored: There are relatively few American grownups who can fit into Japanese Uniforms, and no known Japanese-Americans willing to play the bad guys.

On the other hand, as quartermaster Peterson complained, there are so many volunteer Germans that it's hard to make the battles come out right. "Thaths because German uniforms look better and German weapons are better," said one Grossdeutschlander (whose name dissolved in notes taken with a nonauthentic felt-tip pen). "Also because the German army was the finest and most honorable in the World until the Nazis destroyed it." Yes. Well. But, "Leibstandarte Adolf Hitler"? The elite, fanatic Nazi unit that served Hitler from the 1923 Beer Hall Putsch in Munich through the 1945 Gotterdammerung in the Berlin bunker? "Well, they had the best uniforms and equipment," explained a young man who seemed to know an awful lot about what the LAH troops wore but next to nothing about what they did.

It was let pass; this was just a game, and according to the schedule, it was time to go liberate a keg of beer.

WHERE THE FRONT IS

World War II resumes at noon Saturday when German forces attack an Allied motor convoy on McGlachlin Field at Fort Meade.

The mock battle will be part of the post's Armed Forces Day activities, which will begin at 10:30 with an all-services parade. A precision parachuting demonstration by the Centurions is set for 11. There will also be displays of army equipment old and new, including a MUST field hospital, a vast inflatable space-age version of the old canvas-tented M*A*S*H.

The battle is BYOB (bring your own blanks) for the participants, but the Army will supply the smoke and "beaucoup boom-booms." The Army did not say which side has been designated to win the scenario, but an Allied Forces source pointed out that "this gig is on our own turf."

Fort Meade is best reached by taking Maryland 198 east from the Baltimore-Washington Parkway. The highway becomes Mapes Road bethrough the post and runs right past the parade ground.

Oh, yes: If it rains, the war's off, along with the parade and everything else. This is the New Volunteer Army. CAPTION: Illustrations 1, and 2, no caption; Picture, AN "ENGLISH" PARATROOPER PEERS FROM A BARN DOOR, By Joel Richardson.