Referenda in countries such as Greece and the UK have recently sent shockwaves across Europe because they have presented a popular challenge to international cooperation. This paper examines such high-stakes foreign policy referenda in a systematic fashion. It focuses on referenda in which a potential non-cooperative referendum outcome creates large negative cross-border externalities. Such high-stakes referenda are unusual because the consequences of a non-cooperative referendum outcome cannot be controlled by the national government but instead depends on whether the other countries accommodate or penalize the non-cooperative vote. This implies that voters' expectations about the likely reaction abroad will be highly influential for voting behavior, and that foreign policymakers can influence these expectations by sending costly signals ahead of the vote, thus trying to sway the vote in favor of cooperation. Using original survey data from a recent high-stakes referendum, the 2015 Greek bailout referendum, we show that expectations about the consequences of a non-cooperative vote had a powerful effect on voting behavior. Leveraging the bank closure in Greece, we also show that costly signals sent by the other member states made voters more pessimistic about the consequences of a no-vote and increased the share of cooperative votes.