"They all leave those countries with tears in their eyes," said Master Gunnery Sgt. D.W. Brenna.

He wasn't thinking of Tehran and the Marine embassy guards who got overrun by left-wing guerrillas after only being allowed to fire No. 9 skeet shot and tear gas.

But 118 of the Marine Security Guard School students Brennan oversees were thinking of just that, yesterday, when they filed into the auditorium in Rosslyn to hear the lecture on emergency procedures.

"Who can tell me what action you use before using dedly force?" said the instructor.

A sergeant stood up. "All other methods." The sergeant had his hair cut high and tight, that classic Marine look like he put on somebody else's ears that morning. But here he was, being trained not to use deadly force.

"Who has the primary responsibility for protecting the United States embassy and its personnel?"

"The host government, sir."

The host government? In Iran? In Afghanistan where the American ambassador was killed after being kidnapped by terrorists? What about "first to fight for right and freedom," and all the rest of the Marine Corps hymn?

"Rigidity," said Lt. Col. Donald Knepp, executive officer of the guard battalion. "We call that the drill-instructor syndrome. Those people don't make it through."

It takes six weeks to get about 120 of the Marines' "few good men" and get rid of about 20 percent of them.

"Waterwalkers," one major called them yesterday, citing all the requirements for getting the kind of duty that landed 19 Marines in bad trouble in Tehran: all the top ratings, plus, says Marine Corps Order 1306.2K, promulgated 16 Jul 1975:

"He must be capable of maintaining composure in case of riots or mob action. He must know what to do if a bomb is foun... He will work closely... in some cases with members of the U.S. Army, Navy and Air Force." (That one must hurt.) And moral standards: "Female involvement, excessive use of alcohol and failure to obey lawful orders are the three most common reasons for early relief from the program."

Anyhow, they listened up yesterday, with Tehran on their minds.

"The men know the job itself is crucial, so the morale stays up," said Gunnery Sgt. Vincent Lombardi, whose lap bore a folder blazoned BOMB PLAN with a drawing of a round bomb with a burning fuse, and a little cartoon reflection square on one side.

"What is the purpose of knowing how these plans are set out?"

"Lance Cpl. Snyder, sir: to minimize confusion."

"Exactly."

The United States keeps 1,110 Marine guards on duty in 117 detachments in 103 countries, the largest post being Paris, with 32 Marines, and the smallest including Dublin and Nassau, with six. The guards are not only unmarried, they have to agree to stay that way for the length of their tours, which average 15 months.

"Everybody's ready to go," said Lance Cpl. Brian Vetor, when the subject of Tehran arose during a lecture break. "We're ready to go," said Cpl. Russell Gewin.

And come back with tears in their eyes, as that master gunnery sergeant -- and Marine Corps legend -- has it.

"We take an extra long look at anybody who applies for a second tour," said Lt. Col. Knepp, striding down a hallway on the way to the students' quarters.

"If they enjoyed it that much, then we worry that..."

Knepp has a hard time getting it into words, but the gist is that it's dangerous to give somebody something he likes too much. It's the old Puritan ethic, here in dress blues.

Any lance corporal or corporal is apt to end his tour a sergeant. But then, as Knepp likes to say, "If boot camp at Parris Island is the high school of the Marine Corps, this is the college."

Reveille hits at 5 a.m. so that they may: qualify with the.38 revolver, and fire the 12-guage riot gun for familiarization; get fitted and refitted and refitted for their dress blues; hear lectures on espionage, terrorism, safes and locks, ceremonies, prmciples of leadership and managing funds for everything up to and including -- Semper Fiedlis, after all -- the Marine Corps birthday ball.

But you have to have seen, or actually lived in the squad bays Master Gunnery Sgt. Brennan oversees to understand the real spirit here, the principles of infinite perfectibility in the face of the dreariest Korean War-surplus ambience at Henderson Hall, in Arlington.

"Everything that shines, shines," Brennan says in that tone of voice in which description becomes prescription, or vice versa -- it's hard to tell.

The brass plaque reading Marine Security Guard School shines like "shook foil," as Gerard Manley Hopkins put it. Upstairs, the olive blankets fit the bunks like paint, and, as Brennan points out with his small, cat-like smile, "the laundry bags are empty."

At the guard school, you wash your laundry, you don't put it in your laundry bag -- it might spoil the bag's creases. It's like sending your uniforms to the cleaners -- and when they get back, you press them again yourself.

"Nobody gets to bed before midnight," Brennan says. You can see how you could learn to hate that smile.

"An Irish pennant on a sea bag," says Lance Cpl. Vetor "They got me for that." An "Irish pennant," also known as a "Russian rope," is, in the Marine Corps, the thread, the wisp of lint that may stray from one's uniform.But they ding you for it on a sea bag (elsewhere known as a duffle bag).

Fortunately, the Marines have anodized belt buckles now, so they don't have to Brasso them all the time. Unless they're at guard school, and they're wearing dress blues, which have non-anodized brass buttons. Not that anyone but a Brennan could tell the difference between anodized and brass from more than an inch away -- but don't get caught trying to get away with it.

"My friends back in Illinois ask me what it's like, but if you haven't been in, there's no way you can understand," says Cpl. Patrick Westart, 20, who has selected, as his choice of countries to serve in: Kuwait, Iceland, Poland, Ivory Coast, Boliva.

In that order?

"Be interesting," Westart said, with that opacity Marines seem to cultivate.

"I was in San Salvador, El Salvador," said Brennan. "Was there violence and craziness? It depends what you mean. They were killing priests and kidnapping people. Every time I went to the movies I had to leave word which one. I'd be sitting in my favorite restaurant, Senor Pico's, and the embassy would call -- we were having a recall. Your liverty time was not your own."

It must have been hard to maintain any sort of love life, it was suggested.

It is not for nothing that Brennan has worked for diplomats, you realize as he answers. Or is it that you get the feeling if you were holding a flush, and he backed you down with a big raise, he's the kind that would refuse to show you what he was holding, just on principle.

"Let me put it this way," Brennan said. "They all leave those countries with tears in their eyes."

No doubt saying, with the sweet ring of irony that probably only a Marine can appreciate: "Semper fi!" CAPTION: Picture, Sgt. J. C. Franklin stands guard at the entrance to the Marine Security Guard School; by Gerald Martineau -- The Washington Post