The guerrilla war against the Sandinista regime in Nicaragua is being fought by a shifting motley group of opponents from all sides of the political triangle, including a flood of disillusioned Sandinistas and a wave of American adventurers.

The defectors include such heroes of the Sandinista revolution as Eden Pastora, Arturo Cruz and Pedro Joaquin Chamorro, who were driven from power by a hard-core, tightly disciplined communist minority. These disenchanted Sandinistas have headed for the hills and joined the contras. This has put them in uncomfortable embrace with the Somocistas, who supported the late dictator. They are also joined by the Miskito Indian guerrillas who formerly fought against the Somocistas but are now battling the Sandinistas.

Add to this combustible mix a surge of adventurers from the United States. An increasing number of American volunteers, uncontrolled by the CIA or any other U.S. authority, are turning up in Nicaragua.

My reporter in Latin America, Jon Lee Anderson, has run across several of these expatriate Americans in his travels on both sides of the battle lines. They are, he reports, a varied lot: dedicated political ideologues, cynical mercenaries, naive young tenderfoots and combat-hardened Vietnam veterans. Some are seeking adventure, others fulfillment, others relief from boredom.

A host of Americans have taken sides with the Sandinistas; they live and work in Nicaragua. A few have actually fought alongside government troops against the contras, but most help the Sandinistas by working as planners, writers, economists and translators.

On the contra side, the volunteers have plunged with equal zest into the political intrigues, alley scuffles and jungle ambushes that characterize the counterrevolution against the Sandinistas.

As a random example, 25-year-old George Davis of Great Falls, Mont., was in full combat gear, blazing away at an entrenched Sandinista unit inside Nicaragua, when my reporter encountered him. They spent seven hours together hunkered down behind a large tree with rebels of Eden Pastora's guerrilla group. Sandinista bullets zinged by overhead, killing two men behind smaller trees.

Davis is a tall, blond, rangy kid who had been in the Marines briefly. He said he had come to Nicaragua to "fight communism." After hanging around for three months in a jungle training camp, Davis led the first charge against the Sandinista lines that day. His Nicaraguan companions agreed he had shown great bravery.

But he got an overdose of fun and adventure. He later found work on an American's farm in Costa Rica and earned enough money for his airfare back to the United States.

"John Cattle" is an adventurer of a different stripe. When my associate met him at a Miskito Indian contra camp in Honduras, Cat- tle claimed to be on a "humanitarian fact-finding mission" from a Florida-based foundation. But during a chance encounter on the street in the Honduran capital of Tegucigalpa several days later, the man admitted that "Cattle" wasn't his real name. He said he headed an advance party of mercenaries from an organization incorporated in the Cayman Islands. These soldiers of fortune had come to Central America "on spec," he said, to see if there was any money to be made instructing the primitive Indian tribesmen in the use of sophisticated weapons. But his speculative venture hadn't panned out because the Indians had no money to pay for his team's expertise.

Amateurs or professionals, well meaning or self-serving, the American volunteers are pouring into Central America. One source told my associate of a brigade of 2,000 to 3,000 Amercan volunteers ready to come down and fight.

Meanwhile, the U.S. government is keeping their presence quiet.