This paper explains the post-Cold War surge of nationalism in Southeast Asia and discusses its significance for regional peace and cooperation. As argued, the growth of nationalism as a form of mass politics has different causes in each Southeast Asian context where it occurs, but at the regional level the phenomenon can be explained by three factors: the failure of earlier nationalist movements to fully deliver their promises; a shift in the international and regional order (the end of the Cold War and the rise of China); and a change in domestic order (political liberalisation and democratisation) that was also occurring across many countries in the region. While the main mission of the new nationalism is the defence of national territory, the movements have the unintended impact of bringing together national communities once divided by Cold War ideologies. The phenomenon also poses some serious risks to regional peace and cooperation.