Sudden bursts of violence in Lebanon and Grenada demonstrated more graphically than any military textbook the sort of missions the new U.S. Marine Corps is best qualified to perform-and not perform-around the world.

The country's first taste of real warfare in a decade also sharply underscored the vital role in international policy that a relatively small, once disregarded arm of U.S. military forces now is counted upon to play.

In Lebanon, the bombing massacre of more than 225 servicemen-mainly Marines-by terrorists points up the potentially fatal hazards of using the force for static missions in urban settings.

But in Grenada, the lightning invasion of a remote Caribbean island spotlights the Corps's improving ability to launch swift strikes to protect American interests around the globe.

Not fit for sitting. Experts said the Beirut bombing showed that the offense-minded Marines are ill-equipped for their barracks-bound "peacekeeping" role in Lebanon. They note that their defensive posture on low ground around the Beirut airport left hundreds of troops exposed to the kind of attack that would be unthinkable in a combat situation.

A veteran of duty in Beirut made the point: "The Marines are an offensive force that protects itself by killing the enemy. As long as we're not free to do that, we'll get picked to pieces."

It was a different story on (Grenada. There the Marines, choosing not to land by their usual sea route, descended by helicopter to move swiftly ,with Army rangers and paratroopers and take the island with minimal losses.

Both events-one marked by tears and the other by cheers-spotlight a remarkable turnaround for the 208-year-old service. Some in recent years had written off the Corps as obsolete and too riddled by post-Vietnam ailments of racial tensions, desertion and drugs to be of much use in the 1980s.

A decade ago, in the wake of the American military's losses in a no-win war in Southeast Asia, Marine recruits and those of other services were scoring below normal on intelligence tests. The desertion rate hit an all-time high in 1975. Some feared the Corps would lose its status as an elite force and be reduced to little more than an adjunct of the Army.

But times have changed markedly. Thanks to a pickup in recruiting sparked both by tough economic times and renewed support urged by Presidents Carter and Reagan, the Marines' manpower situation rarely has been better. A Corps that registered a shortfall of 5,000 recruits in 1979 has racked up four straight banner years of enlistments. The force today stands at more than 100 percent strength and is carefully selecting more volunteers among thousands aroused by the latest outbreak of fighting. "Calls have been piling in like you wouldn't believe," reported a West Coast recruiter.

Quality is up as well as quantity. Mental-test scores are rising. The proportion of recruits holding high-school diplomas has jumped from only 50 percent in the late 1970s to 85 percent today.

Equally important, the Marines are in the midst of a major modernization. The President's arms buildup and fresh emphasis on the need for a rapid-reaction force in
the Middle East and other remote areas will materially expand Corps firepower over the next several years. Among the new equipment: Landing craft that skim over
water on a cushion of air and AV-8B "jump jet" aircraft.

Expansion of the Navy's fleet of amphibious ships also promises to enhance Marine mobility.

As experts see it, the leathernecks are uniquely suited to the demands of Reagan's muscular foreign policy. Although their 194,000 number is small when compared with the 780,000-member Army, 558,000-member Navy and 592,000-member Air Force, the Marines take pride in their reputation as the world's best light-infantry forceflexible, mobile and in a perpetual state of combat readiness.

The Corps's basic mission is amphibious warfare-perhaps the most complex, difficult and dangerous of all forms of combat. With their helicopter gunships, tanks and landing craft, the Marines provide the only U.S. force capable of fighting its way into hostile territory from the sea. From their major bases at Camp Lejeune, N.C.; Camp Pendleton, Calif., and Okinawa, the Marines regularly deploy amphibious units to hot spots around the world. They have 436 planes and 102 armed helicopters-one of the world's largest air forces.

Visible roles. More publicly, thousands of Marines serve as security and assault troops aboard naval vessels and stand guard at more than 100 American embassies around the globe.

In addition, the Corps has carved out a new mission as one of the spearheads of the Rapid Deployment Force-now named Central Command-which the U.S. has built to respond to crises in the oil-rich Persian Gulf region. The 12,000man man Seventh Marine Brigade, based at Twentynine Palms, Calif., keeps equipment aboard 18 ships anchored in the Indian Ocean and stands ready to move into action in less than a week.

Through the pain at their losses from the terrorist explosion in Beirut, and pride at their exploits on Grenada, the Marines know as well as anyone that world tensions have cast the spotlight anew on the service at the cutting edge of Reagan's foreign policy.