While scholars have described vertical nation‐building narratives that genealogically anchor a specific group to a specific territory (Smith 1981; Eriksen 2002), I argue that, in addition to vertical strategies, expressions of international solidarity constitute horizontal nation‐building strategies. Expressions of international solidarity can be used to maintain local ethnic boundaries and reinforce local divisions. By adopting an ally, expressions of international solidarity also designate an adversary, making the boundary between the two a possible incentive for solidarity. In Northern Ireland, some Unionist and Nationalist political entrepreneurs rely on expressions of international solidarity with Israelis or Palestinians, respectively, to make adversarial ethno‐national claims to the nation‐state. This study examines flags, graffiti, murals and political speech on display in Northern Ireland that advocate for either Israelis or Palestinians. Through the concept of ‘borrowed legitimacy’, I acknowledge the strategic use of the ethnic boundary in expressions of international solidarity.