The two sweptback Migs zoomed low over the Zambian capital wing tip to wing tip, a new sight and sound in the cold blue southern Africian fall sky that sent nervous residents scurrying to see whether the Rhodesians were on the attack once again.

It turned out that it was only confirmation of what President Kenneth Kaunda had said a few days earlier: "Our boys are being trained to handle sophisticated weapons, and this also involves flying sophisticated jets."

Hardly a week goes by any more without some new signpost-the Migs over Lusaka or yet a worse Rhodesian raid-that the war in Rhodesia is engulfing ever more the neighboring "front-line" states, foremost Zambia and Mozambique.

As much as Rhodesia, these two countries have become part of the general battlefield and increasingly a prime target of Salisbury's strategy to cripple the nationalist guerrillas by knocking out their bases and staging areas as well as their key black African supporters.

Once an easygoing and quiet capital, Lusaka has moved to the top in the rating of State Department hardship posts because of the constant raids, tensions, curfew and partly war-related food shortages.

Western, Eastern and African diplomats here are watching for signs that landlocked and bankrupt Zambia in particular is succumbing to the mounting pressure. Many fear Zambia could feel forced to seek major aid from the Soviet Union, which would place the Rhodesian war squarely into the context of the East-West struggle.

In the coming months it seems a good bet that the new black-led government of Zimbabwe-Rhodesia will concentrate its pressure first on Zambia because of its special vulnerabilities, most notably its growing dependence on the southern rail route that runs through Rhodesia.

That Zambia has endured so far with hardly a flinch the increasing heat of the war is remarkable. Unlike Mozambique, born of a long, hard liberation struggle, this country had never really known war.

For months now, Rhodesian war-planes and helicopter-borne commados have been raiding nationalist guerrilla camps all across Zambia and even around the capital, wreaking death and destruction with virtual impunity.

On April 13, the Rhodesian strategy of taking the war directly to the guerrillas in their rear bases reached a climax with a bold commando raid on the home of Joshua Nkomo just next door to Kaunda's residence here.

"It was a tremendous psychological blow not only to the guerrillas but to Zambia," remarked one European diplomat. "Many Zambians thought it was a coup."

Other than proving the embarrassing inability of the Zambians to defend either their homeland, capital or possibly even the presidential state house, the Rhodesian attack seemed the ultimate provacation to Kaunda to invite in the Soviets and Cubans to bolster both his and the guerrillas' defenses.

Rhodesian propaganda already claims that Soviets virtually run the guerrilla war from this side, with their top African expert and embassador to Zambia, Vassiliy Solodovnikov, acting as chief strategist and his lieutenants sitting in the war rooms of Nkomo's military headquarters.

The objective of all this, the Zambians and Western diplomats here believe, is primarily to convince the United States and Britain that the guerrillas are pawns in Soviet hands and that the West should come to the rescue of the Rhodesian government to counter the spreading Soviet-Cuban involvement in southern Africa.

Despite the provocations, Zambia's tottering economy and its growing economic dependence on South Africa and even Rhodesia, President Kaunda has decided not to fall into the Rhodesian trap by calling in the Soviets nor to give up his longstanding commitment to the liberation struggles of southern Africa.

Significantly, the two MIG19s put on display for the first time in the capital were not of Soviet origin. They were part of a squadron provided by China to Zambia more than a year ago, according to Zambian and Chinese sources.

Though linked by a friendship treaty to Moscow, Mozambique has not shown a significantly different attitude toward avoiding big-power involvement in the Rhodesian conflict, nor have Botswana and Tanzania.

Leaders of all the front-line states have been meeting following the Rhodesian elections partly to discuss how to unite the guerrillas and partly to discuss a common strategy towardthe escalating war, possibly with the military help of other African countries.

Yet the question remains: How long can Zambia hold out?

In military terms, Zambia standing up to Rhodesia or South Africa is a bit like Lebanon doing battle with Israel, so unequal are the two sides' arsenals.

In the end, however, the battle here may well be lost on economic grounds, the Achilles heel of both Zambia and Mozambique.

Top aides to Bishop Abel Muzorewa, the prime minister-elect of the first black-led government of Rhodesia, are saying openly that unless Zambia stops aiding the guerrillas they see no reason why Rhodesia should not close off the southern route.

Ernest Bulle, second vice president of the bishop's United African National Council, remarked at a meeting in late March that "When I look at us allowing the railways to carry his [Kaunda's] foodstuffs to Zambia, I think that good sense will prevail. For surely we cannot continue forever feeding our enemies."

Nor are the threats just economic. Bulle, who served as co-minister of finance, commerce and industry in the transitional government, also warned that unless Kaunda changed its policy, "then one day our soldiers will be dancing in Cairo Road, right in the heard of Lusaka."

James Chikerema, the maverick first vice president of Muzorewa's party, has been even more explicit. He has said in an interview and a public meeting that "the revolutionary prospects" are "now excellent" for ousting President Kaunda.

Murorewa officially has disassociated himself from Chikerema's comments, but what he has told several Western reporters in private does not seem to vary much from what Bulle has been saying publicly.

Current Rhodesian Prime Minister Ian Smith already has tried twice to pressure Kaunda into ending his aid for Nkomo's guerrillas. He refused last October to open the road as well as the rail line between the two countries without a quid pro quo from Zambia on the guerrillas.

In April, after the Rhodesians knocked out the ferry at Kazangulu between Botswana and Zambia, he offered to provide another-provided no guerrillas were allowed to use it to cross between Zambia and Botswana.

The destruction of the Kazangulu ferry severed Zambia's only road route for hauling goods in from South Africa, a growing source of supplies to this country.

Just how vulnerable Zambia is now to Rhodesian pressure became clear in late April when heavy rains washed out the banks of the Chinese-built Tazara railroad to Dar es Salaam, Tanzania, in 18 different places, closing it down. For three weeks, Zambia had just one outlet to the outside world-the southern rail line through Rhodesian.

Even before Tazara was closed, the southern line was taking more Zambian copper out, averaging 30,000 tons a month compared to 25,000 on the Dar es Salaam route.

About the same time, Common Market experts in Brussels were saying it would take more than $1 billion to repair the Benguela line through Angola before it could operate fully again. Zambia once shipped about 40 percent of its vital copper exports on this line. It has been closed ever since the 1975-76 Angola civil war.

It is not just that Zambia is vulnerable by virtue of its tenuous road and rail links to the outside world. Its copper-based economy is depressed and the government owes foreign creditors $1.5 billion to $2 billion.

Shortage of all essentials-from raw materials to keep factories turning to cooking oil to keep Zambian pots cooking-are acute, chronic and not promising to get much better.

Adding to the country's woes, this year drought and uneven rainfall may oblige Zambia to import up to 300,000 tons of maize, the key stapple, to feed the 5 million Zambians. This is roughly half the total marketed consumption.

Meanwhile, mounting crime combined with indiscipline among Nkomo's guerrillas also has led to a crumbling of law and order. It is unknown whether bandits, runaway guerrillas or Rhodesian commandos disguised as nationalists are responsible, but at least nine blacks and whites have been killed in the capital area in the past five months.

In an effort to get a grip on the deteriorating situation, the government slapped an 8 p.m. to 6 a.m. curfew on all major towns and cities that lasted for 27 days.

Amid the mounting tension and malaise, President Kaunda stands determined as ever to confound the prophets of doom. He is going ahead with plans to host the British Commonwealth conference and Queen Elizabeth in August and has just set up for the first time a Ministry of Tourism. CAPTION: Picture 1, Rhodesian commandos destroyed the headquarters of Rhodesian guerrilla leader Joshua Nkomo in residential neighborhood of Lusaka on April 13. Sygma; Map, no caption, By Richard Furno-The Washington Post; Picture 2, This railway bridge at Victoria Falls links Rhodesia to Zambia and is a prime target in the guerrilla war. AP