Some came to honor fallen relatives or friends. Others, such as President Reagan, came in tribute to an ideal: country and duty. A few merely sought a cool place under the trees.

Arlington National Cemetery had room for all of them on Memorial Day, traditionally its busiest day of the year. By day's end, cemetery historian Thomas Sherlock said, 15,000 to 20,000 people would tread its 617 acres of pathways past 192,000 tombstones.

Reagan, accompanied by his wife Nancy, Secretary of State George Shultz and others, laid the traditional wreath at the Tomb of the Unknowns, appeared to wipe a tear from his eye and departed after shaking hands with leaders of veterans groups. His appearance followed a 21-gun salute in honor of the nation's war dead.

Reagan did not speak, but Secretary of the Army John O. Marsh promised that the administration is "fully committed" to accounting for the 2,490 Americans missing in action in Southeast Asia.

Marsh cited the U.S. invasion of Grenada in the wake of Cuban activity there, as well as Soviet support of insurgents in El Salvador and the government in Nicaragua, as evidence of the need for continued vigilance. "Surely it must be perceived there is a growing threat to the collective security of the hemisphere," he said.

The sun gleamed under a cloudless sky and temperatures crept into the 80s during the memorial service at the flag-draped amphitheater, although the heat did not threaten the National Weather Service's May 27 record of 95. Some of the 3,000 spectators protected themselves under umbrellas. Others fanned their programs to rouse a slight breeze.

Marge Smith, of Euclid, Ohio, pressed her hand over her heart as the color guard passed by with flags held aloft to end the 50-minute service. She and her mother, Mary, had left home at 4 a.m. to drive the six hours to Arlington where they would visit the grave of their father and husband, a World War II veteran who died last year.

"It was impressive," Marge Smith said. "Gosh. Pomp and circumstance and military regalia."

Seated in a shady spot at the amphitheater was Emma Green of Arlington, who worked in Army intelligence in the Pacific intercepting Japanese messages during World War II. "The general idea," she said of her attendance at the service, "is to keep alive a remembrance."

Other visitors, too, sounded the theme of the official orders creating Decoration Day in 1868 after the Civil War, for the purpose of "cherishing tenderly the memory of our heroic dead." The holiday, later renamed Memorial Day, now honors all the nation's war dead.

Pat Porter grew up going to military tombs on Memorial Day. Two of her uncles served in World War II, and her husband fought in Vietnam. So yesterday, she drove in from Prince William County to bring two children and a cousin to the nation's best-known war cemetery.

"I'm a firm believer in having children observe the holidays as they should, and go through a bit of uncomfortableness to do it," Porter said. "You don't have the respect you do unless you do that."

Around the nation yesterday, veterans were honored with a 21-gun salute in Boston, and in Rochester, N.Y., three dozen peace groups joined the traditional veterans parade, which had been canceled last year because veterans refused to obey a court order allowing the peace activists to join their march. Beaches and national parks were jammed on the weekend that marks the unofficial beginning of summer.

In the cool trophy room behind the amphitheater, Herb Lintz of Baltimore straightened his medals and joined the Veterans of Foreign Wars contingent for the laying of the VFW wreath at the cemetery. It was one of four dozen wreaths placed by veterans groups on Memorial Day. "We think it's quite an honor," he said.

There is no national memorial to the Korean War although there is growing support for one. Lee K. Humbert of Manassas, part of the American Legion contingent at Arlington, is a supporter. "The Korean is the forgotten veteran," he said. "We didn't raise hell. We didn't come home and complain. We just went about our business."