The danger is that the intrusion of outside powers, with the United States waging a covert war against the Islamists of al-Shabaab, will ignite resentment among Somalis and bolster support for the jihadists.
Some 2,000 Kenyan troops, including the elite 20th Parachute Regiment and supported by airstrikes, artillery and armor, crossed into Somalia Oct. 16 after the kidnapping of several foreign tourists in Kenya blamed on al-Shabaab, which is allied to al-Qaida.
The objective appeared to be to establish a semi-autonomous buffer zone in Somalia to block jihadist cross-border raids that threatened to destabilize Kenya.
The incursion into southern Somalia was aimed at seizing the key transportation hub of Afmadow and the Indian Ocean port city of Kismayo, both important al-Shabaab strongholds south of Mogadishu, and smashing the jihadist organization.
But the offensive has bogged down in winter monsoon weather, while being harassed by hit-and-run attacks by al-Shabaab, which doesn't want to have to fight a pitched battle against conventional forces.
The Kenyans, who've received some $700 million in U.S. military aid this year, boast they can take Kismayo, a smuggling hub and a major source of revenue for the jihadists.
But the hard fact is the Kenyan force needs help itself with little to show for its efforts, including a naval blockade of Kismayo, and no indication it's capable of delivering a knockout blow against al-Shabaab.
Enter Ethiopia, which with U.S. intelligence and logistics support invaded Somalia with a large force in December 2006 to unseat an Islamic regime and replace it with a Western-backed Transitional Federal Government.
Largely Christian Ethiopia, the Horn of Africa's major military power, doesn't want an Islamic state on its southern border while it fights a Muslim insurgency in the Ogaden region. It also wants to keep Washington happy.
Its forces, widely reviled across Somalia as Christian occupiers of a Muslim land, withdrew in 2009 with al-Shabaab's power greatly enhanced by the invasion that was supposed to crush the Islamists.
Western diplomats and counter-insurgency experts say the Kenyan incursion has probably aided al-Shabaab in the same way, and even united its incessantly squabbling factions in the face of an external threat.
A second incursion by the hated Ethiopians will likely have an even greater impact in that regard.
The corrupt and faction-plagued TFG is pretty much as unpopular as the Ethiopians. It's kept in power largely by U.S. funds, a recently built-up CIA presence and a 9,000-strong African Union peacekeeping force known as Amisom, composed of troops from Uganda and Burundi.
The Ethiopians launched their second invasion of Somalia, although on a more limited scale than their 2006 operation, around Nov. 17. It apparently comprises hundreds of troops supported by armored vehicles and heavy artillery who appear to be headed for Baidoa, another al-Shabaab bastion close to the Ethiopian border.
The incursion followed an urgent request for U.S. help from the Nairobi government, which realized it had probably bitten off more than it could chew by pushing into Somalia.
The Americans, who have recently stepped up their covert war against al-Shabaab, are extremely reluctant to engage in another counter-insurgency operation with conventional forces, although they appear to have provided some air strikes to back up the struggling Kenyans.
Washington had warned Kenya it couldn't succeed in dismantling al-Shabaab. The Americans apparently called in the Ethiopians under the cover of the AU to open a new front.
It's a highly charged -- and risky -- return for Ethiopia. Even TFG President Sheik Sharif Sheik Ahmed was unhappy with the Ethiopian thrust.
But he had no choice. The TFG only controls a small part of Mogadishu.
On paper, al-Shabaab appears to be getting squeezed on three fronts – in the east by Amisom around Mogadishu, in the west by Kenya and in the north by Ethiopia and its local proxy, the Ahlu Sunna wal Jama'a militia.
But the current fighting is expected to drag well into 2012, with the threat the 20-year-old conflict will spill over Somalia's borders.