In the midst of civil war, rebel groups often expend significant resources opening offices in foreign capitals, meeting with heads of state, expanding their overseas networks, appealing to international organizations, and contacting foreign media. Existing scholarship has generally neglected international diplomacy as an aspect of violent rebellion, focusing instead on rebel efforts at domestic organization. A systematic documentation of rebel diplomacy in post–1950 civil wars using new quantitative and qualitative data shows that rebel diplomacy is commonplace and that many groups demonstrate as much concern for overseas political campaigns as they do for domestic and local mobilization. Diplomacy, furthermore, is not a weapon of the militarily weak, but a tactical choice for rebel groups seeking political capital within an international system that places formidable barriers to entry on nonstate entities. An original analysis of the diplomacy of the National Union for the Total Independence of Angola in the Angolan civil war using archival sources further demonstrates why rebels may become active diplomats in one phase of a conflict but eschew diplomacy in another. More broadly, the international relations of rebel groups promise to be an important new research agenda in understanding violent politics.