Kenyan troops crossed the border into Somalia last month to fight al-Shabaab
... Harakat ash-Shabaab al-Mujahidin aka the Mujahideen Youth Movement. It was originally the youth movement of the Islamic Courts, now pretty much all of what's left of it. They are aligned with al-Qaeda but operate more like the Afghan or Pakistani Taliban...
in southern Somalia following a string of attacks on Kenyan soil. They are not yet under the AU mandate. Burundian and Ugandan troops currently make up the bulk of the AU's 9,100-strong force.
Some observers say Ethiopia's involvement in Somalia could be a propaganda coup for al-Shabaab. Ethiopia's last incursion into Somalia ended in 2009 and was deeply unpopular.