Threatening clouds and sudden winds swept the sun from Arlington National Cemetery yesterday as a starched and somber Navy firing party paid final respects to Robert Dean Stethem, a gentle Navy man described by his family as a "warrior who refused to let his spirit be broken."

Surrounded by an arch of flowers and hundreds of weeping mourners, the coffin of the 23-year-old diver from Waldorf, Md., took its place in Section 59, just 50 feet from a plaque dedicated to "victims of terrorism throughout the world."

"I just can't understand how we let this happen to him," said Navy Petty Officer Daniel Woodward, who watched silently as pallbearers in full dress whites bore the coffin from the hearse to its final resting place.

"When will there be an end to this?" Woodward asked, as he stood near the graves of dozens of marines killed by terrorists in Lebanon in 1983.

Stethem's relatives sat silently on green velvet chairs as the family's pastor, the Rev. Wendell Cover, delivered prayers over the grave. Richard Stethem, the dead man's father, fought back tears after the brief ceremony when, on hehalf of his family, he expressed "our gratitude to all who have been praying with us."

After the ceremony, the American flag that had been draped over the casket was presented, along with a Purple Heart, to the family.

Stethem's two brothers, Kenneth, 24, a Navy petty officer, and Patrick, 19, who has recently enlisted in the Navy and will begin boot camp in November, saluted their brother's coffin. His sister, Sherry, 27, stood solemnly nearby.

Stethem, who military officials said was beaten beyond recognition before he was shot and dumped on the tarmac of Beirut Airport last Saturday by Shiite gunmen who hijacked Trans World Airlines Flight 847 in Athens, was assigned to an underwater construction team based in Little Creek, Va.

He was returning there after completing a routine repair assignment at a sewage treatment plant at a Navy installation in Athens when he was killed.

About 700 mourners, many of them Navy personnel in blue uniforms and stiff, white caps, crowded the small spot in Section 59 where Stethem was buried. After the ceremony, several officials, including Maryland Gov. Harry Hughes, Sen. Paul S. Sarbanes (D-Md.) and Sen. John W. Warner (R-Va.), offered their condolences to the family.

Earlier in the day, several hundred people attended a memorial service for Stethem at the Word of Life Assembly of God Church in Springfield. Soldiers and friends filed by the gray coffin as the family sat holding hands in the front pew.

"I have to agree with the president," said John Knipple when he heard that President Reagan had said earlier in the day that "our limits have been reached." As he spoke, Knipple stood by the grave of his youngest son, James, who was killed in the bombing of the U.S. Embassy in Beirut on Oct. 23, 1983.

"You have to feel that there comes a time to do something about these horrors," Knipple said. "And I think this is the time."

The ceremony was delayed briefly to allow the mourners -- and the tourists -- time to file into place. During the delay, an eerie light shone down on the group as the sun battled to find its way through storm clouds.

Stethem's brother, Kevin, wept as Navy riflemen sounded a final volley and taps was sounded along the green fields of the cemetery. Both his father and his mother, Patricia, hold Navy administrative jobs and his father spent 26 years in the Navy before retiring.

Friends and neighbors from Waldorf, a small town in Charles County, remembered the dead man as polite and athletic. He was a football player at Thomas Stone High School, and signed up for a five-year hitch in the Navy in December 1980, six months before he graduated.

Long after the funeral ended, after limousines had carried the family away to a more private grief, stragglers -- many of them military men -- strolled up to the grave where headstone No. 59-522 will soon rest. Quietly, and singly, they dropped roses on the coffin and walked away.