Zambia President Kenneth Kaunda left here yesterday for state visits in the United States and Britain at a crucial juncture in his country's involvement in the Rhodesia conflict and his future relations with both Washington and Moscow.

The Zambian President, a central actor in the unfolding southern African drama, is widely credited with having first awakened the U.S. government three years ago to the dangers of its "no policy position" toward black Africa. The message was delivered in a White House speech during his last official visit to Washington in April 1975 and it provoked former secretary of state Henry Kissinger into launching a major American initiative over Rhodesia a year later.

Kaunda is returning to the American capital three years later with another warning, this time regarding the need for swift Western action on the still unresolved Rhodesia settlement issue if an escalation in the Soviet and Cuban involvement there is to be avoided.

In the every near future, the Zambian leader himself must make a crucial decision in this regard: whether to ask Moscow and Havana for increased military assistance to defend his own country against repeated Rhodesian army incursions and to quicken the nationalist guerilla war now aimed at undermining the new multiracial transitional government in Salisbury.

A leading factor in his decision, he has let it be known, will be what plans President Carter and British Prime Minister James Callaghan propose now to force Rhodesia's white prime minister, Ian Smith, from power and the Salisbury govenment to the negotiating table with the Patriotic Front, the guerrilla alliance that is based partly in Zambia.

If Kaunda finds no political will in Washington and London for decisive action, he had been hinting strongly he will turn as a final resort to Moscow even while seeking to preserve Zambia's traditional nonaligned posture.

Such a move could seriously affect his efforts to obtain massive financial and economic assistance in the West - one of the primary objectives of his trip to Britain and the United States - to rescue his country from an acute economic crisis. This at least is what Western diplomats here are suggesting as part of an apparent concerted campaign to dissuade him from turning toward the East for help.

Kaunda is to spend three days in London where he will meet with Callaghan before on to Washington for a two day official state visit starting Wednesday that will include several rounds of talks with Carter.

The Zambian leader has often spoken highly of Carter's human rights campaign in particular. Both men share a deep religious faith. It will be Carter's first meeting with Kaunda, who has long played an important part in trying to find a negotiated settlement in both Rhodesia and Namibia (Southwest Africa.)

In recognition of his efforts, Kaunda is to receive the Martin Luther King Jr. non-violent peace prize while in the United States. He is to visit Georgia, Texas, California and New York.

Kaunda is an excellent barometer of changing moderate African opinion and policy toward the Soviet Union in southern Africa. Probably the most pro-Western of the frontline leaders, the 54-year-old Zambian has found it increasingly necessary to cooperate with Moscow and Havana both to protect his country and built up the Patriotic Front guerrilla army into a credible threat to the former Smith government.

Only two years ago, he was alerting Africans in strong colorful language to the danger of Soviet involvement in the Angolan civil war. "A plundering tiger with its deadly cubs in now coming in through the back door" of Africa, he said of the Soviet Union and Cuba in early 1976.

By sharp contrast, on Friday he had high praise for the developing cooperation between Zambia and the Soviet Union when he met a Soviet Parliamentary delegation on a visit here. "It is our sincere hope that your country will pull hard to wipe out our enemies in southern Africa, Zimbabwe and Zambia where our brothers are being tortured," he told the Soviet officials.

Over the past six months or so, there has been a slow increase in the number of Cubans present in Zambia in connection with the training and support of guerrillas belonging to the Zimbabwe African People's Union (ZAPU), the most moderate wing of the Patriotic Front being supported both by Zambia and the Soviet Union. Western estimates of the total numbers of Cubans here now seem to vary widely from only a few dozens to more than 200.

Perhaps the most visible indication of their expanding presence is the Cuban Embassy, which is said to have more diplomats, attaches and secretaries than that of the United States.

Despite this Cuban buildup, most Western analysts tend to doubt the disposition or need for the Cubans to become directly involved in the steadily worsening fighting inside Rhodesia. Both co-leaders of the Patriotic Front, Joshua Nkomo and Robert Mugabe. have dismissed this possibility and seem to feel it would be ideologically incorrect for outsiders to fight on their behalf.

If the Soviets and Cubans ever do become engaged in the fighting, analysts here believe this is most likely to happen if and when there is an open split between the two wings of the Patriotic Front after the advent of black majority rule in Rhodesia. In that case, they are likely to intervene on behalf of Nkomo's faction against Mugabe's Chinese-backed Zimbabwe African National Union (ZANU). Zimbabwe is he black nationalist name for Rhodesia.

But this analysis does not rule out an increased Soviet-Cuban role in Zambia if no negotiated solution to the Rhodesian tangle is found quickly and particularly if there are more Rhodesian army intrusions like the one of early March deep into Zambian territory. In an interview with the Washington Post just after that raid Kaunda said he no intention of sitting back and watching his country be destroyed and "fail to approach the Russians or Cubans, if they are ready to come, on the ground that Western countries might misunderstand us."

It is generally assumed here that the Soviets and Cubans are more than ready and willing to come to Zambia's assistance and are only awaiting Kaunda's decision on this matter.

The principal Zambian risk in this option, as Western diplomats here are suggesting, is that it is likely to complicate Kaunda's search for massive economic and financial aid in the West to buoy up his country's sinking economy.

Zambia has fallen into a desperate economic straits, partly as a result of its unstinting support of the Patriotic Front and partly of the collapse of the copper market three years ago.

Copper is practically the country's only source of foreign exchange and the government is presently earning vitually nothing from it.

The Zambians have been appealing for private foreign investment to help diversify their copper-centered economy and commodity loans, debt relief and credits to tide them over until copper prices revive. The International Monetary Fund has already come forth with a $400 million loan.

Western diplomatic sources say a large Cuban and Soviet presence in Zambia would probably dampen Western enthusiasm for investing here or aiding the government. But this suggested reaction may be primarily a ploy to pressure Kaunda into not moving any closer to the Soviet camp since the Monetary Fund, Britain the United States and various other Western European countries have already increased substantially their aid to Zambia.