' . . . unto the third and fourth generation'

There is another human tragedy unrolling in these times. In Zimbabwe latest reports say that official government troops have surrounded and sealed off the city of Bulawayo, the capital of the part of Zimbabwe known as Matabeleland.

The leader of the minority opposition party, Joshua Nkomo, is said to be still in Bulawayo, but in hiding. His house has been raided and ransacked. His chauffeur has been killed along with several other of his aides and lieutenants. There are estimates that as many as a thousand followers of Mr. Nkomo have been caught up in what may be a massacre.

There is a current political reason behind the attack by government forces on the organizational center of Mr. Nkomo's political party and movement. The government, headed by Prime Minister Robert Mugabe, suspects that white South Africa may be supplying and planning to use some of the irregular forces whom Mr. Nkomo commanded during the recent civil war and many of whom are believed to be still operating extralegally ''in the bush.''

Some 13,000 of the former Nkomo forces have been integrated into the regular Zimbabwe army, but their loyalty is questioned in Mugabe circles. The suspicion is that they might be tempted to defect from the government and join their former colleagues ''in the bush'' in a South African-financed effort to destabilize the Mugabe government.

There is another and deeper reason behind the friction between the followers of Mr. Mugabe and those of Mr. Nkomo. It threatens to pull the country apart.

In the outside world Mr. Mugabe is usually referred to as the leader of the Zimbabwe African National Union (ZANU). Mr. Nkomo is the leader of the rival Zimbabwe African People's Union (ZAPU). These modern political labels, ZANU and ZAPU, obscure an older and deeper division of the peoples who inhabit Zimbabwe.

When white explorers back in the early 1800s began moving into what is now Zimbabwe, the whole area was inhabited by agricultural tribes who lived relatively peacefully on land cultivation.

In 1830 a tribe of ''warriors and pastoralists in the Zulu tradition,'' known as the Matabele or Ndebele, surged up from the south and (according to the Encyclopedia Britannica account) ''mastered and dispossessed the weaker tribes, known collectively as Shona (Mashona), who were sedentary, peaceful tillers of the soil.''

''For more than half a century,'' says the Britannica, ''until the coming of European rule, the Ndebele continued to enslave and plunder the Shona.''

That account was written when Zimbabwe was still a British colony. It skirts around another facet of the story. The British, also coming up from the south, found themselves in trouble with the Ndebele and fought a savage war with them. The issue was over the right to exploit the Shona. The Ndebele had had a monopoly on that practice until Cecil Rhodes's settlers also wanted to use the Shona.

That issue was settled by the difference between repeating rifles in British hands and spears in Ndebele hands. The rifles won.

Now things are different in Zimbabwe. The British have left. The Shona greatly outnumber the Ndebele. Nearly 70 percent of the population is Shona; some 20 percent is Ndebele. The rest is white. Both Shona and Ndebele have guns. Numbers count. The Shona dominate the government and the official army. Among today's dominant Shona tribesmen is the folk memory of how their grandfathers suffered under Ndebele mastery.

The urge to even the score is human - and can cause deep trouble.

The story is worth knowing because it is so often assumed that all of today's political troubles arise out of East-West rivalry.

Here is a case where the Soviets and Americans are both outsiders. Moscow may hope to benefit from the ancient feud of Shona and Ndebele. The US has a minor interest in pacifying the country. But neither Moscow nor Washington planned or triggered the present surge into Matabeleland of the top units of the Zimbabwe army in the search for suspected ''subversives.''

If there is outside interference behind the scenes, it comes from white South Africa. But basic to the present strife is the condition which existed there between black tribes before the white man came.