POOR AFRICA. In that failed and sad continent Zimbabwe almost alone has inspired hope that things might someday go right. Now that hope is suspect.

The guns are firing. The pluralist political system is endangered. Intimations of fratricide, if not civil war, are abroad. And the cynical definition of African democracy seems once again reaffirmed: "One man, one vote, one time."

That is one view of the violence now wracking the countryside. But it may be based on false premises that lead to overly pessimistic conclusions.

On the face of it, a classic tragedy of tribal politics is unfolding. Government troops rampage with fire and sword through the homelands of the Ndebele people whose political leader, Joshua Nkomo, has been the only credible rival in Zimbabwe to Prime Minister Robert Mugabe of the majority Shona tribes.

By the accounts of journalists and humanitarian organizations, hundreds of Ndebele already have died in this punitive expedition. Hundreds more have been taken into custody, including members of Nkomo's immediate family.

Nkomo has fled under humilitating circumstances to neighboring Botswana, creeping away in the dead of night. If he returns to Zimbabwe he may face serious charges that could send him to prison in his last years. His political party could be outlawed, and the country could face years of armed conflict.

This unhappy prospect may, however, be overdrawn. What is happening in Matabeleland -- the Ndebele provinces -- cannot be explained purely in terms of tribalism or political repression.

On the most basic level, a real issue of law and order is involved. For more than two years, dissaffected and heavily armed veterans of Nkomo's disbanded guerrilla army -- ZIPRA -- have been hell-raising in Matabeleland.

They have brutalized, robbed and murdered shopkeepers and villagers. They have assassinated white farm families on whose labors the commercial agricultural economy largely depends. They have kidnapped and killed foreign travelers, dealing a heavy blow to the tourist economy and damaging government efforts to attract foreign investors.

The serious trouble began when a ZIPRA battalion, once integrated into the national army, staged a bloody mutiny in February 1981 in which more than 300 troopers were killed. There have been subsequent attacks on police and military outposts in Matabeleland. In June of last year, there was an armed assault by former ZIPRA soldiers on Mugabe's official residence in the Zimbabe capital, Harare. This was followed by a series of atrocities aimed at both civilians and soldiers of the national army.

It was against this background that early in January, Mugabe dispatched to Matabeleland the Fifth Brigade, a unit trained by North Koreans and British and dominated by Shona troopers.

The choice of this brigade for service in Matabeleland lent some credence to the charge that its mission was punitive and political. The Second Brigade, based in Matabeleland, was not called up.

Government sources offer this explanation: The Second Brigrade was so decimated by the desertion of former ZIPRA troops that it was unequal to the task. Even the Fifth Brigade, according to these sources, has been suffering defections by its ZIPRA members.

The political overlay to the brigade's operations is complicated, too.

To a considerable extent, the "law and order" problem in Matabeleland is seen by the government as a wave of simple banditry carried out by former ZIPRA guerrillas who have found the good life through the barrel of a gun. But there is also the suspicion that many of the depradations of recent months were politically directed to destabilize the government and disrupt the economy.

One government official offered the theory, backed by "no tangible evidence," that the Soviet Union, Nkomo's principal patron during the war for independence in the 1970s, has encouraged the troubles in Matabeleland. Prime Minister Mugabe's first patron during that war was China, and since assuming power he has dealt at arm's length with the Russians. There is still no Soviet embassy in Zimbabwe.

Yet another suspicion is that the South Africans have stirred the pot and are encouraging dissaffected ZIPRA guerrillas. But again the government claims no hard evidence to support that view.

There is no doubt that Mugabe's troops have played a heavy hand in Matabeleland these last two months. But there is a long history of heavy- handed police actions in the world, and that does not, of itself, support allegations that the operation has a genocidal intent. Nor does it confirm the fear that the one-party state is at hand.

There are two parties to these troubles, and the responsibility for how they are resolved will be equally shared.