Joshua Nkomo, the bulky, jovial 60-year-old father of the black nationalist movement in Rhodesia, has emerged as leader of a Soviet-armed, Cuban-assisted guerrilla army now being readied for full-scale combat with Rhodesia's white-led security forces.

Nkomo's force of about 8,000 trained soldiers is now being augmented by thousands of others who are undergoing training in Angola and Zambia. His final goal, according to African diplomats, is a professional army of 20,000 capable of handling anything from heavy artillery to armored cars and tanks.

Thus in fewer than two years, Nkomo has emerged as a key factor in the military equation in Rhodesia. Apart from getting Soviet weapons and Cuban instructors, Nkomo has the political backing of the Soviet Union, which has already denounced last week's Rhodesian settlement between Prime Minister Ian Smith and three black nationalist leaders.

Already large numbers of guerrillas and heavy weapons have been sighted by Western diplomats moving across Zambia from Angola toward the Zambian border with Rhodesia, in some instances accompanied by Cubans.

With his new, shadowy army behind him. Mkomo has demonstrated a new confidence in his bargaining position. He and other exile leaders have been negotiating with the British and U.S. governments while Smith had reached an agreement with nationalist leaders who live in Rhodesia and do not have armed followers.

But far from showing the political "pragmatism" for which he was once renowned. Nkomo has become steadily more intransigent and committed to a fight to the finish with Smith.

When negotiations between Nkomo and Smith broke down in Salisbury two years ago this March, the black nationalist leader cried from sheer rage and frustration.

"There is no choice left but to fight" he told his supporters. And to reporters, he said "I want to make it very clear that the consequences are not of our own choosing and not of our own making. . . We did everything we could to find a (negotiated) solution."

Indeed, no one could have honestly accused Nkomo of rushing into guerrilla warfare. Since his release from 10 years in detention inside Rhodesia in December 1974, stuck faithfully to the route of legal politicking and, despite enormous risk to his reputation in nationalist circles, agreed in late 1975 to hold talks with Smith.

In answer to critics who accused him of being a power-hungry opportunist, Nkomo said that it was worth trying to negotiate with Smith if only to prove once and for all that it was impossible.

While the more militant rival Zimbabwe African National Union (ZANU) had taken to the warpath in late 1972, Nkomo's own Zimbabwe African People's Union (ZAPU) had held back. In fact, on that fateful day in March 1976 when Nkomo finally admitted he had "reached the end of the road" in attempting to find a settlement through talks. ZAPU had no more than 500 to 700 trained guerrillas.

While an air of mystery still surrounds Nkomo's army, it already has gained a reputation for being better equipped, trained and disciplined than ZANU's still slightly larger and more battle-hardened Zimbabwe African National Liberation Army.

Furthermore, while the ZANU army has suffered thousands of casualties over the past five years at the hands of the hard-hitting Rhodesian security forces, Nkomo's force has lost relatively few because of an apparently deliberate strategy by Nkomo of holding back until his guerrillas are well enough prepared.

How has this incredible transformation in ZAPU's military fortunes taken place in just two years?

The answer lies with the Soviet Union and in the little known history of Nkomo's ties with Moscow and its other black nationalist allies in Africa.

Nkomo's Soviet connection, contrary to appearances, is nothing new. It goes back 15 years or more to the early 1960s when ZAPU was being founded and the Soviet Union was casting about for potential allies among the African nationalist movements in the remaining colonies. Although ZAPU hardly qualified as a Marxist-oriented, or even socialist-inclined party, it was the only nationalist one then agitating in Rhodesia.

At the same time, Nkomo was establishing his ties with other African liberation groups, most notably those fighting in the Portuguese colonies and in South Africa. These included the new ruling Popular Movement for the Liberation of Angola (MPLA), the Front for the Liberation of Mozambique (Frelimo), the PAIGC in GuineaBissau and the African National Congress in South Africa.

For years, Nkomo's Soviet connection lay dormant and he neither sought, nor got, much material assistance from Communist quarters. A man of remarkably little disposition to speak in ideological terms, or even utter the word "socialism." he was far better known for his contacts with British capitalists like Tony Rowlands, the head of Lonrho, whose executive jet was often put at Nkomo's disposal for trips about Africa, Rowlands is still widely suspected of being one of ZAPU's outside financial supporters.

Nkomo's Moscow link endured through the years with the Russians satisfied to cultivate contacts among some middle-ranking officials and a few top Nkomo aides. Also enduring were Nkomo's links to the MPLA and Frelimo as well as his close personal friendship with Zambian President Kenneth Kaunda dating back to their common struggle against British colonial rule.

Today, Nkomo is cashing in on these old friendships and alliances to build himself in record time a military force the equal of Zanu's Chinese-aided one and to create a credible threat to the white-led Rhodeisan government.

His feat is all the more extraordinary since the main source for his guerrilla recruits is the minority Ndebele people, an offshoot of the fierce Zulu living in southwestern Rhodesia. The Ndebele account for only 15 to 17 percent of the country's 6.8 million black population. While the Shona, the backbone of the rival ZANU army, make up 80 percent or more.

But Nkomo has proven himself a master in mobilizing the limited resources available to him both inside Rhodesia and in international diplomacy.

Zambia and Angola are the main staging areas for ZAPU's activities. The triumph in 1976 of the MPLA in the Angolan civil war, largely due to massive Soviet arms and Cuban troops, has turned Angola into an ideal place for ZAPU guerrillas to go for training under Cuban instructors.

There are reports that some recruits for the Nkomo army go directly to Cuba and the Soviet Union for advanced instruction, while others have been sent to military schools in Algeria and Somalia, both until recently Soviet allies in Africa. At least 60 officers are reported to be in Somalia alone. Some officers and soldiers are reportedly learning to drive armored cars, handle heavy artillery and use antiaircraft guns and missiles.

This has led to intense speculation that Nkomo is preparing his army for bigger things than the intensified guerrilla war he is promising these days to launch against the Smith government.

With no sign of any progress being made to integrate ZANU and ZAPU forces into one army, the two armies are likely to confront each other in the struggle for power among Rhodesia's black nationalist factions. Since the Soviets and Cubans are also assuming this, it appears they have decided on a strategy to prepare Nkomo's forces far better than they were able to do the MPLA for any such power struggle.

Indeed, there is every indication the Soviets are applying the lessons of the Angolan civil war to the Zimbabwean nationalist equation, training ZAPU sufficiently well so that Cubans are not required to do the fighting for it as they had to for the MPLA in Angola.

This may explain the sudden increase, to between 25 and 50, in the number of Cubans present in this generally pro-Western country. Most of them are said to be involved in training ZAPU guerrillas in their camps here. Some could become involved in helping to defend these centers, and particularly forward guerrilla camps along the Zambian border with Rhodesia, from the Rhodesian air and commando raids.

All this points to the probability of bigger war in the making between Rhodesia and Nkomo's army. He apparently expects this, too.

"We are going to hit each other hard," he said in an interview last week. "And we are ready for them."