The Romans called it Heliopolis -- the city of the sun--but these days Baalbek is a gloomy place.

It's not just the presence in this former tourist center of unwanted visitors in the form of hundreds of armed Iranian Revolutionary Guards. Nor is the depression due only to the widespread fear that Baalbek may be the next site of Lebanese conflict, since spring increases the likelihood of fighting between the nearby Israeli and Syrian occupation armies.

Rather the gloom is caused by bad news on the hashish front, for Baalbek is the town that hash built.

Hashish cultivation, once confined to less than 10 percent of the narrow Bekaa Valley north of the Beirut-to-Damascus highway, was tolerated by the weak central government. It was an unofficial sop to the poor, backward, mainly Shiite Moslem residents who were neglected when it came to schools, hospitals and roads.

That was before April 1975, when civil war came to Lebanon and the state ceased even pretending to exercise sovereignty over the Bekaa. By last year, farmers had largely abandoned cultivating potatoes, onions and other traditional crops, and as much as 85 percent of the northern valley was sown in hashish.

For the first time since antiquity, when the Bekaa was known as the granary of the Roman Empire, Baalbek became prosperous. New American cars crowded the streets. Shops bulged with television sets, video cassette players and cameras.

Partly because of the prosperity and partly because of Shiites fleeing chronic violence in southern Lebanon, Baalbek's pre-civil war population of 30,000 quadrupled.

But as with so much else in Lebanese life, the new prosperity depended on forces beyond the control of this Connecticut-sized country.

The first bad news was the assassination of Egyptian president Anwar Sadat in October 1981. His successor, Hosni Mubarak, fired the head of Egyptian customs, who had turned a blind eye to shiploads of Bekaa hashish arriving in Alexandria.

Egypt, despite its poverty, was the Bekaa's best customer--taking between half and three quarters of the crop, especially the more expensive grades.

The traffickers were reduced to dumping old innertubes filled with hashish in the Mediterranean and hoping the cargo would be picked up by Egyptian accomplices.

Then the Israelis invaded Lebanon last June. The eastern Mediterranean became crowded with foreign navies--not just the American 6th Fleet and French and Italian squadrons, but also the super-efficient Israelis, whose destroyers and gunboats stopped and searched even the smallest vessel entering or leaving Lebanese waters.

The Bekaa's rich European markets evaporated. No longer did Lebanese dare ship hashish from the Christian-held port of Juniyah or from Tripoli, which is under Syrian Army control. In the past month, the Lebanese government has taken over the port of Beirut from the Christian militia.

As a result, most of the 1982 bumper crop of hashish is still unsold and stored in homes and warehouses in the Bekaa, with no big buyers in sight. This spring, hashish acreage is expected to dwindle to near the pre-1975 level.

Much to many local residents' professed chagrin, as many as 50 clandestine laboratories are said to be refining raw cocaine brought in from South America and heroin base imported from Turkey and Iran.

From what can be gathered from close-mouthed Baalbekis, that traffic passes through neighoring Syria. "We prefer hashish," one grower said. "It is not so dangerous. The heroin and cocaine people are very dirty and very dangerous." But the hard drugs are easier to transport and fetch higher prices.

"You don't light a cigarette here without Syrian permission" is a frequently heard local saying.

Living alongside Syrian troops since 1976--and outside Lebanese government control--the people of the Bekaa have adopted a fatalistic attitude toward these troops, who originally came as an Arab peace-keeping force to end the civil war.

Thus, the presence of as many as 1,000 bearded Iranian Revolutionary Guards in Baalbek and nearby villages is seen as yet another aspect of Syrian influence.

Originally, about 200 Iranian volunteers came via Syria to the Bekaa last summer to fight the Israeli invaders. Their numbers have swollen, but they have done little fighting except against the Lebanese Army.

That at least is the local explanation for the alleged death of one Iranian in a firefight March 4 at nearby Brital, pitting pro-Iranian Shiites (breakaway members of the Amal militia) against a Lebanese Army unit.

Six Lebanese soldiers--and two dissident militiamen--were also killed in the fray, set off by militia leader Hussein Moussawi's objections to allowing the Army to conduct maneuvers nearby.

Brital for years has been a Wild West town, invitingly close to the unguarded Syrian border and long the market for cars stolen in Lebanon and sold in Syria.

Moussawi took exception to the Army on grounds that it was being trained by U.S. marines instead of fighting the Israelis as he felt it should.

Last Nov. 22, Moussawi had celebrated Lebanon's national day by storming the government office in Baalbek, tearing down President Amin Gemayel's official portrait and unsuccessfully attacking the Army barracks there.

His Iranian allies for the most part have been less turbulent--except for the Brital affair--although not as lamb-like as the Iranian ambassador claimed when he inaccurately insisted recently they had been invited by former president Elias Sarkis and were unarmed.

Last month someone blew up the statue of the late Gamal Abdel Nasser, the Egyptian and pan-Arab Sunni Moslem leader, and daubed its desecrated pedestal with Islamic slogans.

Ensconced in two mosques--one proclaimed "Martyrdom Headquarters"--and a rented building on Baalbek's main street, the Iranians have treated Baalbekis to a profusion of poster and mural art extolling Ayatollah Ruhollah Khomeini and condemning his enemies.

"Death to the Americans, death to Israel, death to the Russians," proclaims a large wall inscription next to a powerful fist on a mural. Khomeini's name now graces a central square where the Lebanese police headquarters is located.

Outside their own headquarters, a revolutionary guard with a submachine gun looks out from a sand-bagged position toward the street gate, which is decorated with upended canisters of American-made cluster bombs dropped by the Israeli Air Force.

A revolutionary guard "pirate" radio station daily broadcasts eight hours of revolutionary songs, Islamic propaganda and enthusiastic interviews in Arabic. But few Baalbekis appear to listen.

"We don't like them," a teacher said. "We know about the executions and think they're as crazy as Qaddafi," the colonel who rules Libya. "They are just another occupation army," the teacher sighed. The larger occupation forces--the Syrians and Israelis--were reported to be reinforcing their positions 25 miles further south.

"War is inevitable," said an employe at the Palmyra Hotel, opposite the Roman ruins. It once catered to the rich and powerful of the world but now is all but deserted. Leafing through the book of distinguished visitors, from Kaiser Wilhelm to Charles de Gaulle, he said forlornly, "Sooner or later an Israeli general will sign his name."