The strategy of rebels fighting to overthrow this country's leftist Sandinista government is based in large part on expectations of a short and decisive war.

That is what they promised many of the recruits who swelled their ranks during the past year. Moreover, with the threat that American underwriting will end with the fiscal year on Sept. 30, the "contras" may have little choice but to try to keep their vow made in early spring to march into Managua by late summer.

Yet, while the insurgents look for what one sympathizer in the capital called a "knockout," the Sandinistas have prepared for a test of endurance.

Since the day they marched into Managua themselves almost four years ago, the nine directors of the Sandinista front have built their army and security forces, their political organization, their propaganda apparatus and to some extent their economy with a view that they would be faced with a long challenge. Their studies of other Marxist-oriented revolutions and Nicaragua's own history led to the conclusion that their system would be attacked from within and without.

The fighting now going on in the mountains along the Honduran border near here, in the pine barrens and forests of the Atlantic Coast and in the dark jungles along the San Juan River in the south is all part of one long war, as the Sandinistas see it. It is also one that some U.S. officials and many of the "counterrevolutionaries" often seem hard put to understand.

Talking about what he saw as an increasingly aggressive attitude in Washington, one western diplomat in Managua said ruefully, "Anybody who thinks these guys the Sandinistas are going to walk away, pick up their million dollars in a suitcase and go, is wrong."

A senior U.S. military strategist elsewhere in the region said with mixed admiration and frustration, "We're dealing with people who not only know how to win the insurgent war, but how to capture the revolution after the war. Why can't we capture a revolution for a change?"

Yet American policy makers sometimes tend to equate firepower with victory, even in a guerrilla war. The same strategist suggested that "if the contras can obtain the mantle of inevitable success they can create a popular revolution."

Indeed the purpose of the rebel Nicaraguan Democratic Force and other groups in suddenly taking the wraps off what was long a covert war and the acknowledgment in Washington of what was supposedly secret support appear intended, partly at least, to create that seeming mantle.

The United States, said Sandinista commander Jaime Wheelock, one of the nine, is "artificially inflating a movement that doesn't exist."

"The war we are fighting is against the United States," Wheelock said in a lengthy interview. "It is the United States that moves all the pieces and they are the only ones who give coherence to this picture."

Wheelock said that in addition to arms supplied the rebels by the Honduran Army, which is then resupplied by the United States, the anti-Sandinistas are now receiving what he called "Chinese" AK47 rifles similar to the Soviet and East German arms that the Sandinistas use. Other Soviet-style weapons for the rebels apparently have come from Egypt and apparently from arms caches captured by the Israelis in Lebanon, Wheelock said.

There is some evidence backing this up. The commander of the little Sandinista garrison in the isolated town of Murra at the middle of the Nueva Segovia war zone showed off to reporters last week a small collection of arms that he said was captured from the contras. Among them were AK47 bullets he said were "Chinese." One of these was taken to Managua by a reporter where western arms specialists identified it as Egyptian, manufactured in 1978.

Wheelock also claimed that the contras are extremely well-informed about Sandinista troop movements and suggested they are fed information gathered by U.S. photographic and electronic surveillance.

"We're in an unequal struggle and we will defend ourselves as we have to," he added. But Wheelock concluded that all this "has not weakened the revolution but strengthened it."

Both sides in this war can be accused of wishful thinking. But if there is a single factor that gives the Sandinistas confidence it is their estimation that the forces arrayed against them do not have the ideological or political cohesion needed to sustain a prolonged fight.

The rebels here in the rugged mountains of one of the poorest of Nicaragua's provinces, where men and women who essentially live as pioneers are known for their conservative resentment of any authority, base their hopes for more widespread support on the frequently voiced discontent with the Sandinistas that is heard in the markets and neighborhoods throughout the country.

This disaffection has grown, certainly, with the government's attempts to crack down on dissidents and suspected rebel collaborators in the 13 months since a state of emergency was declared.

But in the cities, which is where the Sandinistas won their rebellion and where the anti-Sandinistas presumably will have to win theirs, unhappiness with the Sandinistas hardly seems sufficient to provoke government-toppling uprisings.

One member of the Conservative Party in Managua, a bitter opponent of the Sandinistas' communist leanings, said that despite its problems "the people don't want for this revolution, as it is, to be destroyed."

The regular troops of the Sandinista Popular Army, the Interior Ministry and the reserves seem deeply indoctrinated and deeply committed not only to the fight against the contras but to the region-wide war for what they see as liberation.

A bulletin board in the Army command center here has the red flag of the Salvadoran Farabundo Marti National Liberation Front painted side-by-side the red and black banner of the Sandinistas.

A reservist wounded by rebel mortars in the border post at El Porvenir said there were no Cuban advisers around that he had seen, but as far as he was concerned "Cuba and Nicaragua are one single hand." As he put it, "Cuba and Nicaragua, Mexico, the Soviet Union, all are peoples who have suffered."

The Sandinista militias, on whom much of the current fighting has fallen, may not be so ideologically firm. But the Sandinistas are working hard, especially in front-line areas like this, to give them something concrete to fight for. There has been extensive land reform in this area, especially in the rich valley around Jalapa. Schools and community centers are going up even in such vulnerable villages as Murra.

A nun sympathetic to the Sandinistas, said the fighting in this area "has conveyed to a lot of people, who are country people, that it's their land. It's theirs to defend. The contra has taught them that."