Despite being proven ineffective as a coercive tool or deterrent, the international community has actively withheld recognition in numerous instances of territorial conquest since the 1930s. Joseph O'Mahoney systematically analyses 21 case studies – including the Manchurian Crisis, the Turkish invasion of Cyprus and Russia’s annexation of Crimea – to explore why so many states have adopted a policy of non-recognition of the spoils of war. By drawing on historical sources including recently declassified archival documents, he evaluates states’ decision-making. He develops a new theory for non-recognition as a symbolic sanction aimed at reproducing common knowledge of the rules of international behaviour.