Somalis say the Ogaden is a "missing" Somali province. Ethiopia's former Marxist government (the Mengistu regime) fought Ogaden rebels (most of the groups allegedly supplied from Somalia). The ONLF has began a "new" low-level war in the mid-1990s.
From Eritrea's perspective, engaging Ethiopia on "the Somali front" may distract the Ethiopian military and reduce pressure around Badme (the contested town along the Eritrean-Ethiopian border).
Ethiopia claims that its troops tracked and engaged a force of at least 300 guerrillas belonging to the Ogaden National Liberation Front (ONLF). The Ethiopian statement said the ONLF fighters entered Ethiopia from Somalia in the vicinity of the town of Shilabo (1,500 kilometers east of Addis Ababa). On April 18, a battle left 32 guerrillas were killed. The Ethiopians also claimed 30 guerrillas were captured. It is believed that the ONLF group was actually supported by Eritrea. Ethiopia claims that Eritrea airlifted the armed group to Dusamarbeer town inside Somalia on Monday and they were then told to cross into Ethiopia with instructions to create havoc and disrupt next month's national election. Ethiopia has elections scheduled for May. Eritrea is quite capable of conducting a covert operation in Ogaden, though this action is anything but covert -- 300 fighters is a big operation in the Ogaden.