With the signing of a friendship and military defense treaty with Mozambique, the Soviet Union has dramatically inserted itself into the fiery politics of southern Africa, taking full advantage of the racial conflicts consuming the region and of the Western powers' failure to find a peaceful resolution to the festering Rhodesia dispute.

The Soviets have carved out for themselves a position in southern Africa and now they are consolidating it," said a Western diplomat here. "I don't see what the West can do about it."

Soviet President Nikolai Podgorny has just completed a nearly two-week visit to Tanzania, Zambia and Mozambique, leaving in his wake a string of joint communiques that emphasized the identity of views between Moscow and these three key "front-line" states on a wide range of African and international issues. The communiques have paved the way for even deeper Soviet involvement in southern African affairs.

His diplomatic voyage was, on the whole, quite successful although it began with one sour note - a Tanzanian public reminder that independent black Africa - has not been getting much Soviet economic assistance - an unscheduled stop in Somalia that highlighted the Soviet Union's suddenly shaky position in the African country where it has made by far the biggest investment.

At times, there was something almost comical about a grandfatherly 74-year old gentleman, sporting a Panama hat and perspiring profusely under the hot African sun, leading a high-powered delegation of 120 high military and government officials, with increasingly dazed expressions on their faces, on an American Express-style tour of southern Africa. Certainly there was nothing particularly revolutionary in their appearance, in sharp contrast to the gun-toting, bearded Fidel Castro, who preceded them.

But Podgorny had long talks with the three African presidents charting the fate of black southern Africa avoided press conferences where he might have made an embarrassing slip, and showed a diplomatic aplomb that belied the old stereotype of the "ugly Russian" stumbling about Africa.

Perhaps the one shortcoming of the trip from the Soviet viewpoint was Podgomy's limited exposure to the public, except in Maputo, where he strolled down the main boulevard with mozambiean President Samora Machel and addressed a public rally.

The Soviet leader's trip was dismissed by many Western diplomats in the capitals he visited as mostly a flat-waving exercise that was long overdue, given the plethora of top-ranking American and British peace envoys who have passed through these same capitals in the past year. Former U.S. Secretary of State Henry A. Kissinger, for example, had already been in Lusaka and Dar Es Salaam three times.

The Podgorny journey was more than a flag-carrying ceremony, however. It brought home to everyone - his African hosts, the white rulers of Rhodesia and South Africa, and the West - the growing importance of the Soviet Union in the course toward spiraling violence that events are taking in this part of the world.

Furthermore, it signaled to all the Soviet intention to consolidate close ties with African nationalist leaders and movements likely to be ruling soon in countries presently under white domination. This policy has already borne Moscow rich fruits in Mozambique and Angola: it could well do so again in Rhodesia, Namiba and South Africa.

The Soviet president got varying receptions in the countries he visited - cordial but unemotional in Tanzania surprisingly warm in Zambia and extremely enthusiastic here in Mozambique. In all three, it was clear that the Soviet Union is becoming regarded as the main outside factor, possibly he decisive one, in the southern Africa liberation struggle and as the only possible protector of the front-line states against white Rhodesian and Southern African agression.

Even the Tanzanians, who showed the least enthusiasm for their Soviet visitor, are now turning toward Moscow to beef up their air and ground defenses.

In short, the Soviet Union seems to have succeeded in making itself indispensable to the front-line states and to the African liberation movements a strong displomatic and military position from which to expand its influence in the future.

Mowcow can now reasonably aspire to become the dominant outside power not only in Mozambique and Angola, with which it has signed friendship and military treaties, but also in Rhodesia and Namibia, once the African nationalists rule there. This could produce precisely the radicalization of southern African that Kissinger was so eager to prevent.

President Kenneth Kaunda of Zambia is perhaps the best bellwether of black-ruled southern African's changingattitude toward the Soviet Union.