Forty years after vast fleets of U.S. and British bombers and troop carriers lifted off daily from the plains of East Anglia to help end World War II, this windswept corner of Britain that juts into the English Channel toward the Continent continues to anchor western defenses.

Perhaps nowhere else among the many U.S. military outposts around the world do the old and the new combine with such continuity.

From the air, one can still see the faint, ghostly outlines of many of the 800 airfields -- some of them only crude landing strips -- that dotted Britain's countryside in 1945.

This was prime real estate for warplanes of what was then called the U.S. Army Air Corps, whose targets were in Central Europe. It still is.

Today, it is headquarters for the U.S. 3rd Air Force, with an array of the most advanced jets, weapons and electronic wizardry. What is here, in effect, is the result of the past 10 years or so of Pentagon developments that finally have made it to the field, some of them late and costing excess billions, but nevertheless now operational.

In the 1940s, it was the Channel that helped keep Hitler's ground armies from overrunning Britain as they had the rest of Europe. Even in today's age of missiles and supersonic jets, the Channel remains an advantage to western strategists worried about the Soviet-led Warsaw Pact.

"That 20 miles of water out there has always been a major factor in history," said the 3rd Air Force commander, Maj. Gen. William P. Acker, "and I submit it will continue to be. It means," in simple terms, "that you can't rapidly overrun this place" as could happen along the East-West front line in Central Europe, where the immediate threat is from huge Warsaw Pact ground armies as well as from air and missile attack.

"England has always been our aircraft carrier, insulating us and the Brits from the first attack on the continent," said Lt. Col. Lee Mazzarella, commander of a squadron of A10 attack jets based at Bentwaters, 50 miles southeast of here.

The range of Soviet Backfire jet bombers makes the concept of a real safe haven in Britain meaningless, said an Air Force planner. But not having to contend with the Soviet Army right away makes Britain the key base for reinforcements and operations if European ports are closed and Soviet ground forces are not being contained on the Continent, he said.

The U.S. Air Force has about 765 planes, 66,000 airmen and women and 63,000 dependents in Europe. About 360 of those aircraft and 30,000 of the troops operate out of the nine main bases of the 3rd Air Force here. The rest are mostly along the front line in West Germany.

In a crisis or in wartime, Air Force plans call for speeding an additional 1,500 jet fighter and attack planes to Europe, including a tripling of the force in Britain. But there is still said to be some uncertainty within the NATO alliance about where those forces would go and whether they would have enough ammunition and fuel to operate, although new long-range commitments were made recently to make more money available.

More than 70 airfields in countries throughout Europe have been designated as Co-located Operating Bases from which U.S. planes could operate in an emergency. But only about 25 percent of these, Air Force sources confirm, have the current NATO minimum requirement of a seven-day supply of fuel and ammunition on hand, enough to fight until more reinforcements arrive.

Some of these bases are not as important as others, and Air Force officials say some are not meant to be fully staffed or equipped. But they say lagging financial support for stocking these bases in both Congress and foreign legislatures is a problem.

In Britain, which is the pivotal air reinforcement center, there are already two large standby bases, and 10 to 13 designated bases that are now used only by the Royal Air Force. Some of the U.S. jets here would also move forward to West Germany. Relations with the RAF are excellent, and the RAF's well-established fuel system for its own 620 combat aircraft eases U.S. fuel problems.

Informants say, however, that an important, unresolved issue now under discussion is how quickly in a crisis RAF combat jets also would move forward to accommodate the huge inflow of more jets from the United States.

Even under ideal conditions of reinforcement, U.S. and allied air forces would be outnumbered 1 1/2 or 2 to 1 by Warsaw Pact air forces. Although allied planes are generally considered superior and their pilots better trained, the U.S. Air Force, in particular, counts on what it calls "force multipliers" -- features that, when added to a force, can provide the effect of having additional aircraft, thereby giving what the military calls "more bang for a buck" from the smaller force.

It is these multipliers that seem to have come together gradually and to be centered here in Britain. They are becoming more important as allied strategy puts more emphasis on hitting deeper behind Warsaw Pact lines in the hope of shutting off second- and third-echelon enemy reinforcements from reaching the front.

Most of the Air Force jets in West Germany are shorter range fighters and attack planes -- 120 older F4 Phantoms, 144 new F16 fighter-bombers and 96 new F15 fighters by last count -- that are meant to disrupt an attack quickly.

It is in Britain that the longer range planes are based, and the vast 3rd Air Force set-up here is designed largely to protect and keep operating seven squadrons of F111E and F attack planes -- about 150 in all.

The swing-wing F111s are the only U.S. planes in Europe that can fly long distances with heavy loads of either conventional or nuclear weapons and find their targets in bad weather and at night.

However, the plane's ability to score bull's-eyes, according to pilots of the 48th Tactical Fighter Wing based here, began to improve significantly about two years ago, when a laser targeting system known as Pave Tack was added to about 84 planes.

"The rate of success is very good," said Capt. Paul Lawrence, a weapons officer. The system allows each F111 to carry four 2,000-pound laser-guided bombs. In the past, one F111 had to carry the laser and beam its razor-sharp light at the target while the other plane's bombs followed the beam.

With the new system, Capt. Mark Zlotkowski explained, each plane can come in fast and low and loft its bombs toward the target while the pilot pulls sharply away so he does not have to pass directly over the target and can avoid air defenses. The Pave Tack system, located in a pod underneath the fuselage, swings and twists like a gyroscope, keeping the target illuminated by the laser beam while the pilot heads for home.

The lasers can be troublesome in rain, but Lawrence claims that the all-weather radar-bombing system allows accuracy to within 200 feet of the target.

Another new development is the EF111. The first of these aircraft in Europe, crammed with electronic equipment meant to jam and confuse Soviet air defense radars, arrived here in February, and there are now five of what will be 12 planes based at Upper Heyford.

Each EF111 is supposed to spew out an electronic screen behind which up to a squadron of F111 attack planes or other A10 jets or the new Tornado attack plane now entering the British, Italian and West German air forces can hide en route to their targets.

The RAF already has about two-thirds of the 220 Tornado strike planes it has ordered, and 420 are entering the West German and Italian forces.

According to the magazine Aviation Week and Space Technology, the electronic jammers on the EF111 are so powerful it is hard to find a place to train the crews. One jammer turned on over California reportedly wiped out every radar up to Seattle.

Equally important as the Pave Tack and EF111s, say Air Force planners, has been the recent arrival here of five of the new TR1 super high-flying reconnaissance planes, the first ones in Europe, equipped with what is called a Precision Location Strike System.

The TR1 is the successor to the U2 spy plane of the 1960s and the 17th Reconnaissance Wing operating out of the base at Alconbury, west of here, eventually will have 12 of the planes, whose space-suited pilots fly above 80,000 feet.

The TR1 is meant to cruise high above the battlefield, so its electronic sensors can "see and listen" far behind enemy lines and pick up precise information on the location of rear-echelon units and relay that information immediately to air strike commanders.

At Mildenhall, south of here, are two super-fast SR71 reconnaissance jets, meant to pick up what the TR1 does not. Using its top speed, three times that of sound, and flying very high, the SR71 is meant to penetrate enemy airspace so fast that defenses cannot bring it down.

Completing the electronic picture being fed to command posts are details on the frequencies and operating patterns of Soviet air defense radars gathered by supersecret RC135 jets based in Britain.

One of the command posts that would get this intelligence in a crisis is a version of the basic Boeing 707 jetliner known as the EC135. The plane is based at Mildenhall and would be used as an emergency airborne command post for the supreme commander of allied forces in Europe, if the ground command posts in West Germany were overrun.

Two of the bases most heavily used here by the United States during World War II were at Greenham Common, south of here, and Molesworth, to the west. Today, they are prime targets of demonstrators against the U.S. presence.

Those two bases are where 160 nuclear-armed U.S. cruise missiles are to be installed (48 are already operational at Greenham). The women who have camped outside Greenham Common for more than a year now have made that base's name a household word.

By all accounts, military relations between the U.S. Air Force and RAF here are excellent, and the conservative governments in Washington and London see eye to eye on the need for a stout defense as public support for the antimissile protest movement has waned.

But opinion polls show that most people still would rather not have the cruise missiles here, and if the nuclear issue bursts to the surface again as it did last year, the tenacious protesters at Greenham Common once again could pose the greatest peacetime threat the 3rd Air Force faces.