The word internationalism originally referred to relationships between nations and states, but came quite early to mean relationships of solidarity between people and peoples across or despite national boundaries, inter-state conflicts and economic competition. Over the past few centuries it has been a constant feature of social movement practice, from the 1649 Leveller mutiny against joining Cromwell’s campaign in Ireland to the 1984 – 87 Dublin shopworkers’ strike against handling South African produce - or contemporary international solidarity with struggles in Mexico, Kobane or Ferguson. International solidarity has been hugely important in changing the terms of politics. External supporters often provide crucial sources of legitimacy,...