The contributors analyse the key roles in constructing peaceful international couple relationships and their foreign-policy outcomes, at a time of acute trust deficit in international affairs. They show that to establish prospects of conflict transformation and sustainable international policy cooperation, the most positive long-term impact derives from sub-state intermediary levels and middle-class elites promoting integration through incremental identity-change, rather than from diplomatic engagement between rivals (despite the relevance of leaders embedded in institutional frameworks for facilitating rapprochement). A differentiation approach is applied throughout the volume, revealing the various mechanisms at work, moving from the rationalist to the sociological, from intergovernmental strategic bargaining to societal integration and the construction of new identities below the state-level.