The states of the Horn of Africa have never fully subscribed to the doctrines incorporated into the Charter of the Organisation of African Unity, prohibiting intervention in the internal affairs of other African states, and promising respect for existing national frontiers. Even though those doctrines were themselves set out at the OAU’s founding conference in Addis Ababa, in a document drawn up under the aegis of the government of Ethiopia, they reflected the tactical interests of that government in making common cause with other African states, rather than any commitment to the principle of non-intervention itself. Every government in the Horn has been deeply engaged in the affairs of its neighbours, and none (save possibly the microstate of Djibouti, which has lacked the means to do so) has ruled out the use of military force as part of that engagement. The recent history of intervention has thus reflected, not the breakdown of previously respected conventions of state sovereignty, but a continuation of the complex interactions between political forces in the region that reach back decades, indeed centuries, into the past. This chapter will concentrate on the governments which seized power in both Addis Ababa and Asmara in May 1991, bringing about in the process the separation of Eritrea from Ethiopia, and its emergence two years later as a recognised sovereign state. It will be especially be concerned with the war between Eritrea and Ethiopia that broke out in May 1998, and shattered the good relations between the two countries that had at least ostensibly been achieved since 1991, setting in train a new round of regional instability. A preliminary historical survey is nonetheless essential.