In the southern bushland of this African nation, rebel leader Jonas Savimbi is building camouflaged treetop platforms from which to fire new U.S. Stinger missiles at Soviet-built fighters and bombers.

Hundreds of miles to the northwest, at Huambo and Lubango, Angolan government pilots -- under the tutelage of Soviet and Cuban advisers -- are flying day and night training missions in Soviet-built Mi24 assault helicopters and MiG23 jet fighters.

Both sides are preparing for what may become the largest clash ever in Africa's hottest bush war.

Savimbi's South African-supported army already claims to have shot down, in the past two months, 10 Soviet-made aircraft, including jets, helicopter gunships and one Antonov 22 transport in heavy skirmishing in the south central highlands. There is no way to confirm these claims independently.

Meanwhile, officials of the Marxist government in the capital of Luanda are hinting that they may strike -- under the doctrine of hot pursuit -- at Savimbi's support bases and supply lines in neighboring Zaire, from which the Central Intelligence Agency supervises U.S. support for Savimbi, according to Angolan officials and other sources in Washington.

As both sides maneuver their armies into position, the threat of a widening war is becoming a major concern in the region.

It has been learned that officials from Mozambique, the Angolan government's closest African ally, recently brokered a secret meeting in London between a top Savimbi lieutenant and an emissary from Luanda in hopes of getting the warring parties to the negotiating table. Angolan officials deny such contacts took place.

These exploratory talks showed no visible result, but they demonstrated that even the pro-Soviet Mozambicans believe the time has come to give Savimbi's guerrilla movement a share of the power in Angola.

For its part, the Reagan administration -- in pursuit of a tougher anti-Soviet stance in Africa, and with renewed aid to Savimbi -- is stepping onto a war-weary battlefield, one where two large and well-equipped armies, totaling more than 150,000 men, are poised for another bloody -- and this time critical -- confrontation.

The pressure on the Angolan government either to crush Savimbi or negotiate with him has never been greater.

The country's $3 billion budget is financed almost exclusively by oil revenues, which have been halved by the rapid decline in world prices. Luanda's desperate financial plight has called into question whether the government can continue to pay for the war.

The stakes for Savimbi are equally high as he seeks to prove that he is a good bet to bring home the first victory for the Reagan administration's anticommunist struggle.

During two weeks in July, two Washington Post reporters visited the rival camps, talked with their leaders and traveled to front-line areas to assess the battlefield preparation, military strategies and impact of this civil war in which the United States has staked a new claim.

Among the strongest impressions taken away was the extent to which Angola is disintegrating as a functioning state.

There are thousands of land mines sown in abandoned corn fields, from which hundreds of thousands of peasants have fled to the cities, making the once self-sufficient country now heavily dependent on food imports.

There are no reliable casualty figures for this war. No one knows the civilian death toll.

In Luanda, residents trade soap, cigarettes and bottles of beer for fresh fruit and vegetables because money has become meaningless: barter is the principal means of trade.

Road travel outside a 50-mile radius around the capital has become so dangerous that it is not safe to travel without a platoon of heavily armed escorts.

The once-thriving diamond industry in the northeast has been ravaged by repeated guerrilla attacks and now is a wasteland where bandits and Savimbi's guerrillas ply the river beds for gems to buy food and guns.

In government-controlled areas, villagers say Savimbi's guerrillas are terrorizing them -- planting mines in their gardens, dragging off their youth and indiscriminately shooting civilians -- as part of a brutal campaign to demonstrate that the central government cannot protect them.

Although both sides claim military advantage, there is an overwhelming sense of unending stalemate here. The end of each rainy season brings government columns down from the north, riding on Soviet armor to assault the guerrilla redoubts in the south.

And each year the columns bog down in the sandy bushland terrain until the attrition of guerrilla hit-and-run attacks sends them limping back to their northern bases.

Although there are no precise figures, it is estimated that Savimbi's forces number about 40,000, made up of both organized guerrilla units and dispersed fighters, while government forces are estimated at 110,000 regular troops and militia. There are also an estimated 27,000 Cuban soldiers here supporting the government and another 8,000 to 9,000 Cuban advisers. Civil War to Superpower Conflict

The fighting began as a civil war that flashed 11 years ago with the sudden departure of Portuguese colonial power. But it quickly developed into a superpower contest, and in the end -- after the victory of the Soviet-backed Popular Movement for the Liberation of Angola (MPLA) and U.S. withdrawal of clandestine aid, in January 1976, from Savimbi's National Union for the Total Independence of Angola (UNITA) -- it became the rock on which U.S.-Soviet detente foundered.

Angola since has smoldered as a distant war, a symbol of Soviet gains in southern Africa and disdain by Congress -- which forced the end of aid to Savimbi upon a reluctant White House -- for covert American involvement in Third World quagmires.

Now, President Reagan and his national security advisers have decided to reenter this regional conflict in the name of the "Reagan Doctrine," a catch phrase used by conservative supporters of Reagan for the president's determination to back anticommunist "freedom fighters" around the globe.

In March, the Pentagon and CIA began shipping sophisticated American weapons to Savimbi's guerrillas. The aid flowed after Savimbi and his supporters in the American conservative movement mounted a lobbying assault on Washington last February that included a 15-minute meeting and press photo opportunity with Reagan in the Oval Office, counseling at the Defense Department and political coaching -- under a $600,000 contract -- from a well-connected public relations firm, Black, Manafort, Stone and Kelly in Alexandria, Va.

Buoyed by this success, Savimbi said in an interview that he now intends to return to Washington this fall and seek an even larger commitment than the initial $15 million CIA support program approved last year by Reagan. In doing so, Savimbi warned that U.S. oil companies operating in Angola would be attacked if they opposed his bid for increased aid from Washington.

American involvement with Savimbi is increasingly apparent. In early June, a delegation of staff members from the Senate Select Committee on Intelligence slipped secretly into Angola, escorted by a U.S. Marine lieutenant colonel, to evaluate Savimbi's military organization.

Before they returned to Washington, the staffers met with CIA station chiefs in Kinshasa, Zaire, and Pretoria, South Africa, as part of their oversight mission.

The outsiders in this war are the South Africans, the Soviets, the Cubans and, now, the United States, and their conflicting strategic interests, along with continuing military aid, provide the means for this war to go on indefinitely.

After more than a decade of struggle, each side is still too weak to inflict a decisive military blow on the other or to hold a significant amount of the vast Angolan territory, which is twice the size of Texas.

The government controls the larger towns and cities and exercises loose control over two-thirds of the country. Savimbi operates freely in the remaining one-third and has infiltrated thousands of guerrillas deep into northern Angola. Morale High in Rebel Camp

In Savimbi's camps, morale is high and the imprint of South Africa is strong. Everything from soft drinks, soap and cigarettes to trucks, weapons and gasoline is shipped overland or flown to clandestine airstrips from South African bases across the southern border in Namibia.

Savimbi has been bracing for a new offensive since last winter, when he warned Reagan administration officials that he could be wiped out if he did not get sophisticated weapons to blunt the onslaught of Soviet tanks and armored helicopter gunships arrayed against him.

The offensive did not materialize in April, as Savimbi had predicted in Washington, but by late May the opposing forces had clashed in the central highlands between the railway towns of Luena and Munhango. The fighting started just weeks after Savimbi had brazenly flown two planeloads of journalists to Munhango, his birthplace, to show that he operated with impunity well north of his southeastern strongholds.

Apparently in response to Savimbi's grandstanding, several columns of Angolan government troops launched a pincer movement to capture Munhango, kicking off this year's offensive hundreds of miles from where it was expected to begin.

The government troops scored an initial victory by taking the hamlet of Cangumbe in the last days of May, but then Savimbi, who claims to outnumber the attacking force in the area, counterattacked and brought the offensive to a standstill, inflicting heavy casualties on the government.

In an interview, Savimbi maintained that the Angolan government's 30th Brigade, which spearheaded the assault on Munhango from the west, has been devastated -- its numbers reduced from 2,000 soldiers to 600 by casualties and desertions and all of its tanks and armored cars destroyed or disabled.

Savimbi's control of Munhango is still threatened from the east by another armored column inching forward down the railway line from Cangumbe.

In Luanda, there is an information blackout on the fighting around Munhango. It was Savimbi's forces who announced that the government had taken Cangumbe.

Diplomats and relief agency officials confirmed the heavy fighting, which has forced the suspension of International Red Cross emergency relief flights from Huambo to villages in the central highlands.

For Savimbi, the confrontation in the center of the country was a carefully orchestrated feint, the rebel leader said in an interview. By daring the government forces to come after him at Munhango, Savimbi said he had hoped to force the government to pull reinforcements from the south, where Savimbi fears the main thrust of an offensive this year.

He claimed partial success. Angolan Army commanders detached forces from several government brigades in central and western Angola to reinforce the battered armored columns trying to take Munhango, Savimbi said. However, they left intact the main force poised for a southern thrust toward Savimbi's strongholds at Mavinga, Likua and Jamba.

"My plan is working," Savimbi said. "I knew that by going to Munhango that they the government would come and try to occupy it."

Now, Savimbi claims he has tied up so many government troops trying to take Munhango that he has effectively eliminated one of the three attacking forces available to march southward into his "liberated territory" when the main offensive begins.

Savimbi said he has hard intelligence that a debate is raging inside the Luanda government over whether to attempt another southern offensive or instead try to drive his guerrillas out of the heavily infiltrated northern provinces. Last year's southern drive failed after eight weeks of heavy fighting and the rout of government forces at the Lomba River near Mavinga, 150 miles northwest of Jamba.

Savimbi said he had reports that the central government was under pressure from the Soviets to score big military points against his forces in the south this year. Luanda Avoids Baiting South Africa

In Luanda, there are indeed signs of governmental hesitation but primarily because officials say South African Army units have moved into southern Angola to block any new attempt to dislodge Savimbi.

In an interview, the Angolan Army chief of staff, Col. Antonio dos Santos Ndalu, said two South African battalions are more or less permanently stationed now in the Ondjiva area, 25 miles from the Namibian border. In addition, he said, South Africa has moved the so-called Buffalo Battalion, a mixed, 3,000-man force of UNITA and South African troops, into a defensive position northwest of Mavinga to protect the approach to Savimbi's headquarters at Jamba.

Backing up these elements are an additional 20,000 regular South African troops based just across the border in Namibia and ready to launch their own offensive at any moment, Col. Ndalu said.

"They can do it whenever they want. But they generally do it when we prepare to launch an offensive against the puppets," he said, referring to Savimbi's forces.

One important element in the government's war strategy this year is to halt the steady infiltration of Savimbi's guerrillas into the northern provinces from Zaire. The region is of particular importance because it is the country's main coffee-growing area, and the government has launched a campaign to boost exports, hoping to take advantage of high world coffee prices.

When the president of neigboring Zaire, Mobutu Sese Seko, landed at Luanda airport July 10, he was given the traditional African warm greetings complete with dancing troupes. But the words he heard from his Angolan government hosts over a luncheon during his brief five-hour stay in the capital were not so cordial.

"We are convinced UNITA is using Zaire today," said Vice Foreign Minister Venancio de Moura in an interview. "We don't want to resort to the right of hot pursuit like South Africa, but our military has had enough.

"Zaire was informed we have evidence they UNITA guerrillas are being trained in Zaire," he said. "We cannot cross our arms and wait for Zaire, South Africa and the CIA to destabilize Angola."

But the fact that Angola and Zaire were meeting to discuss the new tension in their relations was a positive sign, de Moura said. Mobutu had admitted nothing but had gone back home promising to "investigate" the Angolan evidence presented to him, he added.

Soviet pressure on Luanda to score military points against Savimbi and Savimbi's thus far successful strategy to delay and divert an offensive against his southern headquarters leave uncertain the course of this war and its outcome in this critical year.

Next: Boom and bust in Angola