Washington — The next few weeks and months could be decisive in the struggle for Central America. In the latest development, a new antigovernment force has announced that it will soon launch its first military operations inside Nicaragua. Alfonso Robelo Callejas, a leader of the Costa Rican-based Democratic Revolutionary Alliance (ARDE), says that his group has been forced to take up arms because Marxist-Leninists now control Nicaragua and have betrayed the original goals of the revolution.
Mr. Robelo is allied with Eden Pastora Gomez, better known by the guerrilla name of Commander Zero, a hero of the Nicaraguan revolution who gained worldwide publicity when he and his men briefly captured the National Palace in Managua in 1978.
According to Robelo, Mr. Pastora is now inside Nicaragua. Some American analysts say that this group has a better chance of mobilizing popular support against the Sandinistas than do the CIA-backed forces now fighting in the north of Nicaragua. Other analysts are not so sure that Pastora can score any significant successes given the tight grip the Sandinista regime holds over its armed forces and the way in which it has expanded and mobilized these forces.
But Commander Zero is only one factor among several in what could become a rapidly changing situation in Central America. According to a US State Department analyst, within the next six months:
* Pressure on the Sandinistas from the Pastora group and from CIA-supported forces in the north could produce an incentive for the Sandinistas to reduce their dependence on Cuba and East bloc nations and negotiate over their differences with neighboring countries.
* The ''relative success'' of the Salvadorean guerrillas in moving forcefully into areas of El Salvador where they have shown little strength before could end if the new military high command in El Salvador can organize itself effectively. According to the analyst, the guerrillas have in recent months benefited from the ''immobility'' of the Salvadorean Army. That immobility, he said, was due to a power struggle within the Salvadorean high command, which resulted in the ouster of Defense Minister Jose Guillermo Garcia. With a new defense minister in place and additional American aid coming, the Salvadorean Army may be able to take the initiative, the analyst said.
Another factor may work against this optimistic scenario: Key Democratic members of the House Intelligence Oversight Committee are moving to cut off covert funds provided through the CIA to groups operating in and against Nicaragua. The committee is scheduled to vote on the measure Tuesday.
Inside Nicaragua itself, a Sandinista military officer reported his troops had clashed on April 28 with rebel soldiers under Pastora's command. The officer said that his troops killed two rebels and captured 40 others in an area not far from the border with Costa Rica.
In Washington, Robelo said he knew of no such clash and that the report of ARDE's men being captured was not true. Robelo, a former member of the leftist-led Sandinista junta in Nicaragua, said that ARDE forces had an unplanned clash about two weeks ago when they ambushed a Sandinista boat that had been tracking them. Two of the Sandinista troops were killed and six were wounded, he said.
The bearded Robelo, an American-trained chemical engineer who made a fortune in Nicaragua in agribusiness, indicated that it was impossible for the antigovernment forces now inside Nicaragua to overthrow the Cuban- and Soviet-backed Sandinista government unaided. He said that ARDE was not asking for materiel assistance from the US - and would not accept any help from the CIA - but that it was urging the US to continue to put diplomatic and political pressure on the Sandinistas.
''The ideal situation would be to put enough pressure on them so that we can sit at the bargaining table to find a solution . . . to save Nicaragua from the destruction of war,'' said Robelo. ''But we have to be careful when we talk with them. We don't want to be fooled by them again. We need international guarantees.''
Robelo met here on April 29 with Adolpho Calero Porto Carrero, a leader of the Nicaraguan Democratic Force (FDN), the CIA-backed anti-Sandinista group now fighting inside Nicaragua from bases in Honduras. Robelo said that ARDE at some future time might cooperate with the FDN but that for the moment the only thing the two groups agreed on was that the main obstacle to peace in Nicaragua was ''the Marxist-Leninist regime'' there.
''The FDN is not our enemy,'' said Robelo, who argued that contrary to common belief in the US, the FDN forces do not include a high percentage of former Nicaraguan national guardsmen. The guardsmen, who fought for the late strong man Anastasio Somoza Debayle until their defeat in 1979, are detested in many parts of the Nicaragua.
''We want to rescue the Nicaraguan revolution and put it back on its original track,'' Robelo said.