South Africa's early-morning commando raid on suspected guerrilla bases in neighboring Botswana today has dealt another body blow to the Reagan administration's already weakened regional policy of "constructive engagement."

It capped weeks of increasing violence in this simmering region that has also included new at-tacks in South Africa itself, neighboring Mozambique and Angola. And it illustrated how escalating violence inside Africa's last white-minority-ruled state can spill over its borders to endanger even the most stable of its black neighbors.

For months Reagan administration officials fighting a rear-guard action against antiapartheid demonstrators and congressional critics have invoked the argument that their policy has succeeded in measurably lowering the level of violence in southern Africa.

"The southern Africa region now has less violence than at any time in the past 10 years," said an unnamed senior American diplomat earlier this year, a statement echoed repeatedly by State Department officials. They contend that U.S. policy, by seeking warmer ties with all countries in the region, including South Africa, has played a role.

But recent events, including the raid, have undermined this argument and been used by critics who contend that constructive engagement has given South Africa a freer hand in threatening its neighbors.

Two weeks ago Angolan forces killed two South African commandos and captured a third inside Angola just weeks after Pretoria announced that all of its troops had withdrawn from that country, which borders the South African-held territory of Namibia. South Africa said the commandos had been gathering intelligence against anti-South African guerrillas operating from Angolan territory, but Luanda presented evidence indicating the commandos were planning to sabotage oil installations jointly owned by Angola and Gulf Oil Corp.

Earlier this week Angola's Marxist government accused South Africa of preparing for a new invasion of its territory. A government communique claimed South Africa has massed four motorized brigades and 15 battalions totaling 20,000 men along the Angola-Namibia border, supported by 90 fighter planes and helicopters. Pretoria has not commented on the report.

Escalating violence in Mozambique between the Marxist government and guerrillas of the Mozambique National Resistance movement has caused more than 100 deaths in the last two months, according to news reports from that embattled nation. It has led President Samora Machel to request new military assistance from his closest African allies, Zimbabwe and Tanzania, in an emergency summit here two days ago.

In South Africa itself the homes of two politicians were attacked with hand grenades this week, assaults that Pretoria said were carried out from Botswana by guerrillas of the banned African National Congress (ANC) and that led to this morning's retaliatory raid. Nearly 400 people have been killed since last September in black unrest, according to official figures, and South African police are poised for further violence this weekend, the ninth anniversary of the 1976 Soweto uprising.

All of these incidents have their origins in the internal situation inside South Africa and Namibia. The attacks on Angola are in large part a response to that government's harboring of insurgents of the South-West Africa People's Organization, which has fought a two-decade bush war to oust South Africa from Namibia. Similarly, Pretoria for several years funded and guided the Mozambican rebels to punish Machel's government for harboring members of the ANC.

Official support for the Mozambican rebels was supposed to cease last year following the signing of a nonaggression pact between Pretoria and Maputo. The fact that the rebels are still operating in many parts of Mozambique is a blow to that pact and to the United States, which helped broker the accord.

To honor the accord, Machel forced his longtime ANC allies to abandon his country a base of operations, driving them to seek other points of entry into South Africa. Because of its long, sparsely guarded border with South Africa, Botswana has become a prime replacement despite efforts by that government to avoid being entangled in the conflict.

South Africa has conducted several cross-border attacks in recent years against insurgents based in Mozambique, Angola and neighboring Lesotho. But today's raid marked the first major assault against Botswana, long considered the region's most politically stable state with one of its smallest armies.

The raid followed two bomb attacks in Gaborone, the Botswanan capital, in recent months by unknown persons against black South African refugees believed tied to the ANC, and it is likely to incite fears that Botswana could become this region's equivalent of Lebanon, torn between opposing outside forces stronger than itself.

The South African raid came only days before ANC leaders are scheduled to hold their first full-scale congress in 16 years at an undisclosed location in the region. It may exacerbate the split in the movement between those arguing for and against an immediate mass insurrection in South Africa. An ANC manifesto issued in late April called for an intensification of armed struggle and renewed work stoppages to "make the country ungovernable" but backed away from calling for a general revolt.

The timing of today's raid could not be worse for the Reagan administration, which is attempting to stave off congressional passage of economic sanctions against Pretoria.

"It's a death blow to constructive engagement," said Maj. Michael Evans, professor of military history at the University of Zimbabwe. He said the raid showed that Pretoria's military strategists still hold the upper hand over its diplomats in determining regional policy.