Each side accused the other of starting the battle -- an artillery, tank, missile and infantry engagement fought between trench lines on a scrubby patch of land called Badame. Each side also claimed the other suffered heavy casualties.

Independent observers had not reached the remote battle site, but a series of urgent -- and ultimately futile -- international peace efforts were driven by fears of the level of destruction that all-out war likely would bring. Hundreds died in a series of pitched battles on the border last June, and since then each side has poured tens of thousands of troops into a front that now runs along the entire 600-mile border.

"This is the beginning, definitely," said a senior relief official in Addis Ababa, the Ethiopian capital.

The stage was set by angry official rhetoric that reached its highest pitch in the last month and by recent statements by Ethiopian officials that the war could begin "within days."

In the Eritrean capital of Asmara, where Ethiopian MiG-23s bombed the airport in June, police cars equipped with loudspeakers cruised the streets urging residents to stay indoors.

Ethiopian Airlines meanwhile moved its flight operations from Addis Ababa, setting up shop 700 miles south in Nairobi just hours after fighting broke out.

"We find that very significant," said Yemane Ghebreab, a senior Eritrean official in a telephone interview from the Eritrean Embassy in Washington.

"We just want to be very careful," said Haile Kiros, an Ethiopian government spokesman.

Fighting was confined today to a 15-mile section of the most heavily contested stretch of border, a triangle of land northwest of the ancient Ethiopian city of Axum (a historic site that Ethiopian officials recently claimed Eritrea had plans to sack). Both the genesis and the outcome of the battle were in dispute.

Ethiopia issued a statement claiming that Eritrean troops launched a "major offensive" on its position at Badame -- but also reported that while "chasing Eritrean forces, [Ethiopian forces] captured Geza Gerlase, a major Eritrean stronghold."

Eritrea's state-owned news agency insisted the offensive was Ethiopia's and claimed that hundreds of Ethiopian soldiers lay dead in the no-man's land beyond the trenches from which they charged. Ghebreab, in Washington, said Eritrean forces had captured 100 prisoners of war and put two brigades of 1,500 to 2,000 Ethiopian soldiers "out of action."

"Our positions are defensive, and will continue to be defensive," said Yemane Ghebremeskel, an adviser to Eritrean President Issaias Afwerki. "Our position is, Do we need this war?"

Not on paper. Ethiopia, with a population of 60 million, has more people than all but one other African nation, and 20 times more people than Eritrea. But that tiny nation, once a province of Ethiopia, won its independence in 1993 after a 30-year war that ended with the defeat of the behemoth -- albeit with the help of a second rebel movement that went on to form Ethiopia's current government.

Moreover, on the way to independence Eritrean forces captured a huge share of the arsenal the former Soviet Union lavished on Ethiopia's then-Marxist government. Even when relations between neighbors were exceptionally friendly and each was demobilizing its armed forces, Issaias said that his military was equipped for a nation five times its size.

Since the border dispute erupted, however, both sides have gone on arms-buying sprees. In December Ethiopia spent a reported $150 million on used Russian fighter planes, helicopters and other arms. Eritrea has purchased light arms by the planeload, as well as an undisclosed number of MiG-29 fighter jets.

The arsenals support forces that Western analysts estimate at more than 200,000 troops on each side.

The stage has thus been set for a war rarely seen in Africa: a conventional, cross-border war between sovereign states. More common on the continent are low-intensity, guerrilla conflicts involving rebel groups, which are sometimes supported by neighboring states.

But today's fighting broke out with the latest peace initiative still pending. U.N. special envoy Mohamed Sahnoun was in Addis Ababa today, where he had traveled from Asmara in the latest attempt to broker peace. The U.N. Security Council has endorsed an Organization of African Unity plan that calls for Eritrea to withdraw from territory it occupied in May and submit the border dispute to arbitration.

The United States has maintained close ties with both countries, which President Clinton cited before the current hostilities as exemplars of the so-called "African renaissance." U.S. officials said diplomats were working "in high gear" behind the scenes to arrest the fighting and steer both sides toward direct negotiations.

"They've both been told that neither side can win decisively," said Ted Dagne, a Congressional Research Service analyst who follows both countries closely. "It just doesn't make sense. This is madness."