By Martin Plaut
BBC Africa analyst
The war in Somalia has increased in intensity, with Ethiopian planes bombing the airport in the capital, Mogadishu.
Somalia's government troops are getting help from Ethiopia
The city is the stronghold of the Union of Islamic Courts (UIC), which are now confronting the Ethiopians in locations across 400km (250 miles) of Somalia.
The Ethiopian incursion, in support of the weak but internationally-recognised Somali transitional government, is using tanks and heavy artillery, as well as aircraft and is reported to be making significant gains against the lightly-armed Islamic forces.
Addis-Ababa has made it plain that it is determined to end what it sees as the threat from Somalia's Islamists once and for all.
In a televised address, Ethiopian Prime Minister Meles Zenawi said: "Our patience was considered as weakness and we were forced to go to war and the alternative left to us is to speedily bring the war to a successful and victorious end in the shortest time possible."
Ethiopia has one of Africa's largest, best-equipped and most experienced armed forces, with more than 100,000 trained personnel.
But the war they are now facing in Somalia will test their abilities to the full.
The eastern region of Ethiopia, through which its forces must travel, are remote and have few resources. Its supply lines and communications were stretched even before they crossed the Somali border.
It is also an area inhabited by Somali speakers, some of whom are sympathetic to the UIC now controlling Mogadishu.
Somali and Oromo rebels operate in the area, and there have been reports of skirmishes in recent weeks.
Inside Somalia the Ethiopians are likely to find few allies in the area they are now entering.
Many Somalis, who are united by nothing else, will be determined to resist Ethiopian forces, with whom they have fought two wars in the past 50 years.
But the greatest challenge for Ethiopia is that its army is now facing hostile forces on two fronts.
Eritrea, with whom it fought a bitter border war, that ended just six years ago, is fully mobilised and heavily armed.
Somalia's Islamic militiamen have declared jihad against Ethiopia
Ethiopia has to maintain a strong presence along its northern border, while fighting a hot war with Somali Islamists in the east.
This comes at a time when there has been some discontent within the Ethiopian army.
Several senior Ethiopian army officers have defected to the rebels based in Eritrea in the last few months.
None of this implies that Ethiopia is incapable of meeting the challenges posed by the war in Somalia. But there are risks for Prime Minister Meles that he will have contemplated before committing his forces to the conflict that is now under way.