COLOMBIA'S NEW PRESIDENT, Andres Pastrana, promises to stem the flood of cocaine his country pours into the United States at the rate of 700 tons a year.

He sounds better than his predecessor, Ernesto Samper, who took $6 million from the Cali drug cartel to help finance his 1994 election campaign. But stopping the narcotics traffic is easier said than done.Colombia supplies 80 percent of the world's cocaine and has become a major exporter of high-grade heroin. It is both a grower and processor of Andean coca leaf and opium poppies from Bolivia and Peru, refined in jungle labs under the protection of left-wing guerrillas.

Arms bought with drug profits have enabled the rebels to wage Latin America's longest-running insurgency, lasting 34 years and winning control of half the country. Drug money also lines the pockets of corrupt politicians and creates a climate of violence in which narco-traffickers thrive.

Colombia leads the hemisphere in murders and kidnappings. Between them, left-wing rebels, right-wing death squads and army troops have killed 35,000 people and frightened more than a million into fleeing their homes. Massacres occur almost daily in rural areas where rebels and paramilitaries, the latter tacitly supported by rogue army units, collect "war taxes" and target peasants they believe are sympathetic to their enemies.

Pastrana will have to make peace with the Revolutionary Armed Forces of Colombia (FARC), whose strength is estimated at 15,000; the smaller National Liberation Army , about 5,000 strong; and nine right-wing death squads containing 4,000 paramilitary soldiers allied under the umbrella of the United Self Defense Forces of Colombia.

The guerrillas earn more than $800 million a year from drugs, extortion and kidnapping. A Colombian security service report published in July listed FARC's income at $463 million in 1997 ($312 million of it from drug-running), and the ELN's at $341 million ($200 million from drugs, $42 million extorted from the oil industry and $98 million from the coal-mining sector).

Colombia's respected news magazine Cambio 16 said rebel income had grown by one-third during Samper's four-year presidency. In the same period the government tripled its defense budget to $2.65 billion, without any measurable improvement in the 120,000-man army.

It has received significant American support since President Clinton lifted Samper-inspired sanctions earlier this year. Washington is providing military training, helicopter gunships and intelligence assistance, despite warnings that it is being drawn into a Vietnam-style conflict with no light at the end of the tunnel.

In March, Gen. Charles Wilhelm, commander of the U.S. Southern Command, acknowledged that Colombia's armed forces are incapable of defeating both the guerrillas and drug traffickers. He blamed "weak national leadership, an overloaded and often corrupt judiciary and the ineffectiveness of the security forces," whose performance "provides little cause for optimism."

Pastrana, educated at Harvard, once kidnapped by the Medellin Cartel and the man who blew the whistle on Samper's ties to drug traffickers, has convinced Washington that he cannot stop the flow of cocaine without ending the insurgency that has plagued Colombia since 1964.

After winning his country's presidential election, he had a jungle rendezvous with FARC's legendary leader, Manuel Marulanda, and secured promises of peace talks from both guerrilla groups and the paramilitaries. But the rebels staged a devastating offensive just before his inauguration, killing 143 soldiers and police and capturing 133 others.

But the rebels say they still want to talk peace - just on their terms.