To clear up any misconceptions, the United Nations–as a rule–does not send peacekeepers to places where there is no peace to keep. Somalia today certainly falls into this category.
Peacekeepers are trained to keep the peace, not mount invasions. Furthermore, the Secretary General does not have any standing forces at his disposal. When the Security Council approves a peacekeeping mission, the Secretary General must rely on member states to pony up troops and equipment. To complicate matters, member states are generally reluctant to offer their troops for a peacekeeping mission that has no ceasefire or political agreement to uphold (see: Sudan, Darfur).
The Security Council can, however, approve the kind of mission that Alex Thurston considers necessary to save Somalia.The defense of Kuwait in 1990 and Australia’s interventions in in East Timor, for example, were authorized by the Security Council. However, these are not “UN peacekeeping missions,” but essentially war-fighting efforts led by individual member states. For humanitarian intervention to occur in Somalia tomorrow, an individual country, NATO, or some coalition of the willing would have to take on the project themselves. Presumably, this would include evicting Ethiopian troops, suppressing an insurgency and defeating spoilers. So far, no country seems willing to take this on, so the next best option is to work to secure a political agreement between as many factions as possible and then use UN peacekeepers as the guarantors of that peace. The newest Secretary General’s report on Somalia, linked here, recommends this path–and I suspect the Security Council will approve.