LA PAZ, BOLIVIA -- U.S. officials have long said their strategy of attacking the drug trade in the Andean cocaine-producing nations has its best chance of quick success here in Bolivia, and have backed up that conviction with a growing commitment of manpower, technical aid and money.

But now, perhaps on the verge of breakthroughs, the anti-drug fight here is up against the same circumstances that make progress so difficult in neighboring Peru and Colombia -- entrenched corruption, growing sophistication on the part of drug traffickers and the first signs of political terrorism.

As a result, the anti-drug fight here, as elsewhere in the region, is being seen as a long-haul proposition that will require years of attention and resources.

"On the whole, the effort is going as well as can be expected," said a U.S. diplomat involved in the anti-narcotics war, "bearing in mind that it is a problem that cannot possibly be solved without protracted effort involving a number of years of patience."

The most immediately worrisome new factor is the emergence of political violence directed against Americans, highlighted by an Oct. 10 bombing at the residence housing the U.S. Embassy's contingent of Marine guards.

There were no U.S. casualties, but a Bolivian policeman who was guarding the house was killed and another was injured. Two men pulled up in front of the residence and staggered out of a car, pretending to be drunk. They then opened fire on the two Bolivians stationed in front of the house and set off a powerful bomb that destroyed the front gate. "Miraculously," according to an embassy spokesman, it did not injure the three Marines inside.

U.S. officials here see the attack as part of a string of incidents stretching back to the Aug. 8, 1988, bombing of a motorcade carrying then-Secretary of State George P. Shultz on the outskirts of La Paz.

In May 1989, two Mormon missionaries were assassinated. Around last Christmas, the U.S. Embassy was bombed. On the same night that the Marine residence was bombed, another explosion toppled a statue of John F. Kennedy that had stood in a La Paz square.

"There is a qualitative difference between the political terrorism here and that in Colombia and Peru," said U.S. Ambassador Robert S. Gelbard. "Here, Americans are the primary target, as we've seen over the past couple of years."

In Colombia, violence sponsored by the Medellin and Cali cocaine cartels and by political groups on the left and the right claims thousands of lives each year, and puts some parts of the country effectively beyond the reach of law enforcement.

In Peru, anti-narcotics efforts in the Upper Huallaga Valley -- source of more than half the world's coca, the plant from which cocaine is made -- are consistently thwarted by the presence of Shining Path guerrillas.

A previously unknown leftist group called the Nestor Paz Zamora Command -- named after President Jaime Paz Zamora's late brother, a former guerrilla who fought against Bolivia's military rulers two decades ago -- claimed responsibility for the Marine bombing. U.S. officials also have been threatened by a right-wing group calling itself the Consorts of Death, believed to be sponsored by drug traffickers.

Neither group is believed to be large or to have popular support, but U.S. officials are taking the threats and incidents seriously and have revamped security procedures.

In Bolivia, coca is grown in a region on the eastern slopes of the Andes called the Chapare and then flown north to the lowland Beni and Pando regions for processing.

The U.S. strategy here is to disrupt that flow, cutting off the peasant growers from the traffickers who buy their coca leaf. The idea is to interfere with demand by blocking the access of coca to the market, thus driving the price of coca so low that it makes sense for growers to take advantage of government programs that pay them to switch from coca to other crops.

Earlier this year, U.S. officials were sure they had succeeded. The price of a 100-pound bale of coca had dropped from its August 1989 high of nearly $90 to just $11 in April.

Farmers were rushing to take advantage of programs under which the Bolivian government paid them $2,000 cash for each 2.2-acre parcel of coca they switched to other crops, plus $6,000 in U.S.-backed credit assistance. Some officials believed they had been successful in pushing the coca price below the farmers' break-even point.

However, in what U.S. officials call a seasonal fluctuation -- and what other experts call the market's return to normalcy after absorbing a glut-like bulge in the coca supply -- the price of a bale of coca shot back up to nearly $50 in September. Now the price is said to hover around $30.

Farmers have voluntarily converted more than 15,000 acres of coca to other crops this year, although U.S. officials acknowledge that some growers secretly plant new coca fields on other land under their control while at the same time gladly taking the government's conversion money. Nevertheless, Gelbard said he is convinced that this year Bolivia will see its first-ever net reduction in coca acreage.

U.S. Drug Enforcement Administration agents, working with the Bolivian anti-narcotics force UMOPAR to attack trafficking networks, had what they describe as their biggest success to date in late September with a series of raids on an organization allegedly headed by Carmelo Dominguez, described as one of the top half-dozen traffickers in the country.

Dominguez had been attracting attention with his large purchases of coca paste, a substance produced as one of the intermediate steps on the way from coca leaf to finished powdered cocaine. Officials say he was working to fill a large order placed by the Colombia-based Cali cartel, which is trying to take advantage of the growth opportunities presented by Colombian authorities' continuing campaign against the violent, once-dominant Medellin cartel.

In coordinated strikes, DEA and UMOPAR arrested Dominguez and his top aides and pilots, and seized nine airplanes, two hangars, a trove of records, nearly $300,000 in cash and a discotheque headquarters Dominguez allegedly owned in Santa Cruz, a booming lowland city that is home to many of Bolivia's major traffickers.

During that raid, DEA agent Hawthorne Hope was shot -- the first action-related casualty in the DEA's expanding Andean drug war. Hope is now convalescing in the United States. In a follow-up raid, UMOPAR and the DEA arrested a suspect in the shooting.

The recent raids, while hailed as a breakthrough, also illustrated the increasing sophistication of the Bolivian drug trafficking rings. What once were thought of as mom-and-pop, shoestring operations now have airplanes, advanced communications equipment and high-tech laboratory facilities for refined cocaine.

Like the governments of Peru and Colombia, that of Bolivia has been less than enthusiastic about U.S. efforts to enlist Andean armed forces in the drug fight.