Eritrean troops are fighting inside Ethiopia

The British media company BBC reports on Thursday that eyewitnesses in Tigray have seen large numbers of Eritrean soldiers in the town of Adigrat, which is located near the border between Ethiopia and Eritrea.

- I saw around 40 trucks with soldiers. The convoy was moving south, an eyewitness told BBC news in the local Tigrinya language.

The soldiers were said to have been heading towards the interior of Tigray to meet the federal Ethiopian army which had advanced north from the provincial capital Mekelle. Last week, the airport outside Mekelle had been taken over by the TPLF rebels, who are fighting the government in Addis from their base in Tigray. But the government side retook Mekelle after an offensive over the weekend and then continued north.

The fact that Eritrea's army is now demonstrably participating in the war in Tigray means that the neighboring countries are once again in open conflict for the first time in 26 years. Between 1998 and 2000, Ethiopia and Eritrea fought a war that was followed by two decades of political ice age. The relationship improved greatly since Abiy Ahmed came to power, and just four years ago Eritrea's army fought on Abiy's side against the TPLF rebel group.

But in recent years, the relationship has deteriorated sharply, mainly due to Abiy's aggressive tone regarding Ethiopia's need for a port on the Red Sea.

Eritrea has now changed sides and is backing the TPLF. Eritrea in turn is assumed to receive support from Egypt and Sudan, but the question is in what form and to what extent.

On paper, Ethiopia's army is likely stronger than both the TPLF and Eritrea combined. But support from the outside world can change the outcome, as happened during the war in Tigray four years ago.

In addition, Abiy is challenged internally not only by the TPLF but a total of seven different rebel groups who have made common cause to try to overthrow his rule.