But despite Eritrea's peaceful accession to independence in 1993, the boundary between the two countries was never properly defined.

Patrick Gilkes, an expert in the area: basic contradiction in the way Ethiopia and Eritrea are governed

Over the past five years, the Eritrean and Ethiopian governments have argued peacefully about the border - originally defined at the time of Italy's colonial occupation. But a series of recent events have exacerbated the tensions

• Last November Eritrea introduced its own currency, damaging cross-border trade.

• There have been longstanding tensions over the loss of Ethiopia's main port - Assab - when Eritrea became independent.

• In May border tensions increased with both Ethiopia and Eritrea accusing the other of occupying parts of their territory around the border.

Former allies

The Eritrean foreign minister pores over colonial maps

Eritreans and Ethiopians fought side-by-side for years to overthrow the previous Soviet-backed dictatorship in Addis Ababa, paving the way for Eritrea's peaceful accession to independence in 1993.

This new conflict threatens the current fashionable assertion, frequently advanced by the US administration, that a new and visionary group of ex-guerrilla leaders is transforming Africa.

Ethiopia's President Meles Zenawi and Eritrea's Isayas Afewerki, brothers-in-arms during their long rebellion, are members of this group along with the leaders of Uganda, Rwanda and the Democratic Republic of the Congo, formerly Zaire.

Historic rivalry

The US Assistant Secretary of State for African Affairs, Chester Crocker, also points to a historic rivalry between the Tigrayan Ethiopians, who are now the dominant ethnic group in Ethiopia, and the Eritreans.

US State Department Chester Crocker says Ethiopia is a Tigrean regime

"The people who are fighting each other should not be viewed as all of Ethiopia against all of Eritrea. It's really Tigrayans and Eritreans going at each other," said Mr Crocker.

The two groups are traditionally very close, and the Ethiopian and Eritrean leaders fought together to overthrow the military dictator Mengistu Haile Mariam in 1991.

But Mr Crocker says that there is also rivalry between the two peoples, which may underlie the personal antagonism between Meles Zenawi and Isayas Afewerki.

Military balance

Ethiopia's population of 60 million outnumbers Eritrea's by 15 to one. But Eritreans have been hardened by a 30 year war of liberation and have a formidable fighting record.

Paul Beaver of Jane's Defence Weekly discusses the two sides' firepower

Analysts suggest Ethiopia could be badly bruised if it came to an all-out conflict. The economic effects could also be severe if Ethiopian businesses lose access to Eritrean ports.

Both countries retain large standing armies an the area is awash with tanks, artillery and aircraft provided by the former Soviet block.

Outside interests

Both Ethiopia and Eritrea have received considerable backing from Washington as part of its policy to pressure the Islamic government in Sudan.

Eritrea officially supports Sudanese rebels. During its short history as an independent state, Eritrea has come close to war with Yemen over islands in the Red Sea and has squared up to tiny Djibouti over another dispute.