There was an item in the Reuters news feed about an attack in Northern Kenya by al Shabaab guerrillas on a Kenyan police patrol, killing 5 and setting the truck on fire.  The story was essentially context free with the explanation given:

Al Shabaab has also launched other attacks in Kenya targeting civilians, in revenge for Kenya moving its troops into Somalia in late 2011.

The deployment of African Union troops to Somalia as peacekeepers (really peace enforcers since there is no peace to keep) is not really the background of this story.  Somalia is principal irredentist state (now failed state and hole in the political map) in Africa.  Somalia from the start challenged the inviolability of borders enshrined in the Organization of African Unity Charter (now the African Union).  At decolonization, the newly created African states were multi-ethnic.  In fact, Somalia is the rare example of a nearly ethnically homogenous state.  Patriotic sentiment was lacking, states were weak, and therefore, it was instrumental that states agreed to respect each other’s borders in the OAU.  Somalia rejected that position, arguing that all Somalis should be living in a greater Somalia.  It even triggered the Ogaden War between Ethiopia and Somalia (1977-1978).

There are large numbers of ethnic Somalis in Northern Kenya and ethnic unrest has been endemic, spurred on by Somali nationalism.  It is ironic though that Somali nationalism is a strong cultural force, there is in fact no Somali state any longer due to another stronger cultural force, kin-based clans, which have warred for control of the territory for decades now without any victor emerging, hence the fundamental hole in the political map that is the failed state of Somalia.