The International Seafarers' Federation (ISF) was a global union federation bringing together trade unions representing sailors. Until the end of World War I, sailors were represented by the International Transport Workers' Federation (ITF), but unions representing sailors became increasingly convinced that the ITF was dominated by the interests of dockers and railway workers. The leadership of the National Union of Seamen (NUS, active in the UK) and the International Seamen's Union (ISU, active in the United States) were also unhappy that the ITF was increasingly sympathetic to socialism, and both unions left the international in 1917.