At first, you think he is just a traffic cop, sent to keep commuters from roaring too fast through Sheridan Circle, a traffic roundabout on Washington's Embassy Row.
But eventually, if you pass by often enough, you realize the ''policeman'' never writes a ticket, and always stands at the same place - where 23rd Street peels off and heads toward Georgetown.
In fact, he is not a policeman at all. He is a Secret Service agent in uniform, protecting the nearby Turkish Embassy against Armenian terrorists. Ironically, his post is only yards from the spot where Chilean politician Orlando Letelier was killed in 1976 by a powerful car bomb.
The guard is a subtle reminder that terrorism is not limited to such cities as Beirut and Belfast. It can strike in the United States, even on this mansion-lined street where limousines are as common as Toyotas.
Over the last decade, the US has been hit by relatively few terrorist attacks. But recent events - the destruction of US facilities in the Mideast, the November bombing of the US Capitol - have convinced many government officials and outside experts that the US must guard against an upsurge of terrorism on its own soil.
''There are some very nasty people out there who would love to think they can intimidate this government,'' says Roger Young, FBI assistant director and spokesman.
At the most likely terrorist targets in Washington, signs of a new wariness abound:
Visitors to the Capitol are much more carefully screened, and portions of the building have been closed to the public. Some key legislators, such as Sen. Charles H. Percy (R) of Illinois, chairman of the Foreign Relations Committee, and Sen. Howard H. Baker Jr. (R) of Tennessee, the majority leader, have stripped the ''US Senate'' license plates from their cars.
Reporters entering the White House grounds can no longer just flash their passes and breeze by the guardhouse. They are stopped and scrutinized by metal detectors. Waist-high concrete slabs now block the White House's southwest entrance, to prevent vehicles from smashing through the gate.
(The seven-foot fence that surrounds the White House, however, is not exactly baling wire and balsa wood. Two years ago, this reporter saw a small car careen down Pennsylvania Avenue, turn right, and try to batter through the northwest gate. The car hit the barrier and stopped dead, as if it had run into the side of a mountain.)
Ironically, these precautions are going up even though terrorist activity in the US has steeply declined since the late '70s. In 1977, according to the FBI, there were 111 terrorist incidents in the US. In 1982, there were 51; through November of last year, terrorists had struck here 29 times, killing 6 people.
Slightly less than half of last year's attacks were bombings, estimates the FBI's Roger Young. Of these, the most spectacular was the election-eve explosion in the Senate side of the US Capitol. An Army Reserve Center in the Bronx, the Washington office of the Soviet airline Aeroflot, and three Cuban-owned offices in Florida were also bombed.
The rest of the year's incidents were a grab bag of assaults, robberies, and hijackings. The deadliest terror crime of 1983 occurred in North Dakota, where two federal marshals were killed by the shadowy, right-wing group Posse Comitatus.
Those responsible for these attacks are definitely dangerous people. Compared with their counterparts in other parts of the world, however, terrorists now active in the US are, in essence, minor leaguers.
They usually are members of small groups, with limited resources and a narrow political purpose often unrelated to the US. For instance, the group responsible for the most terror attacks last year (five) is Omega 7, a band of fanatical anti-Castro Cubans.
Other groups now active here include the Armed Forces of National Liberation, dedicated to the ''liberation'' of Puerto Rico; and the American branch of the Secret Army for the Liberation of Armenia, pledged to avenge an alleged massacre of Armenians by the Turks in 1915. (Armenian groups, though small, are considered among the world's most skilled terrorists by many who study the subject.)
All in all, terrorism ''is a major problem here, but not nearly to the degree it is in some other countries,'' says Mr. Young of the FBI.
Then why the sudden chill in the air in Washington? Are the new antiterrorist precautions simply a skittish reaction to what's happening overseas?
Young and other terrorism experts reply that the new watchfulness is prudent. They list three main reasons why:
* Even though the number of attacks has declined, terrorists may still be out there, plotting. Terrorist activity is cyclical, they say, and a quiet '83 may mean groups are gearing up for a hot '84.
* Hardened radicals from the Vietnam war era may be reentering terrorism.
Alumni of the violent Weather Underground have been linked to a series of recent crimes - including the 1981 robbery of a Brink's armored car, blasts at New York military installations in 1982, and the recent Capitol explosion.
''I think you'll find the Capitol bombing was done by a guy who was active in the late '60s,'' says a congressional intelligence source. ''It's the same people, a little older, a little meaner.''
* Most important, the major leaguers of terrorism - state-sponsored gangs, mainly from the Mideast - may expand their operations to the US for the first time.
Many governments (notably the Soviet Union) have a history of covert backing for terrorists. But increasingly, notes a State Department analyst, such countries as Iran, Syria, Libya, and North Korea are using terrorism as simply another foreign policy tool.
As the devastation of the Marine compound in Beirut made all too clear, the US is one of the prime targets of this government-sanctioned terror, say experts on the subject.
''The $64,000 question is whether Middle-Eastern groups will hit over here. My bet is they will. I think there will be some incidents,'' says Dr. Robert Kupperman, a Georgetown University professor with ties to Israeli intelligence.
A knowledgeable congressional source says he has no hard evidence indicating such an attack is imminent. But he adds that US intelligence has identified a number of US residents ''with a Middle-Eastern background'' who have received terrorist training overseas.
The FBI is worried that a number of 1984 events - the Los Angeles Olympics, the New Orleans World's Fair, the Democratic and Republican conventions - may attract terrorists as well as tourists.
But terrorists, by definition, do not strike in a predictable manner.
The Olympics, World's Fair, etc., will be ''hard'' targets, with fairly tight security, points out Dr. Kupperman, a senior associate at Georgetown's Center for Strategic and International Studies. He says it is more likely terrorists will hit ''soft'' targets, such as relatively unprotected federal government buildings.
To emphasize the FBI's belief that terrorism here remains a serious problem, Director William Webster last year raised terrorism investigation from a ''Priority 3'' to a ''Priority 1'' program. A 45-agent Hostage Rescue Team has been founded. Guidelines governing investigation of potentially violent groups have been changed.
Important as these steps are, the government will probably never be able to guarantee 100 percent protection against terrorists.
''What the terrorists would like most is for us to overrespond, turn this country into a police state,'' says bureau spokesman Roger Young.
''That's the last thing we're going to do. [The US] will probably accept a degree of vulnerability to avoid a diminution of freedom.''