This thesis examines the Women’s International League (WIL) to explore the wider themes of feminism, pacifism and transnational activism during the Great War and the interwar years. WIL was formed in October 1915 as the British national section of what came to be known as the Women’s International League for Peace and Freedom (WILPF). The thesis interrogates the concept of feminist pacifism by analysing WIL’s campaigns for peace, disarmament and international law alongside its pursuit of women’s rights. The thesis also demonstrates the interplay between activism on the local, national and international stages. In chronological terms, the focus is on the first twenty years of WIL’s activism: from the circumstances that led to its foundation in 1915 to the challenges faced in 1935 – a time when the political consensus within WILPF came under threat and when hopes that internationalism would secure peace began to fade. The study comprises five chapters. The first explores the foundations of WIL and examines the methods it used to link its opposition to war to its feminist demands. The remaining chapters are thematic and cover the organisation’s work during the interwar years. Chapter Two analyses WIL’s campaigns for women’s rights, including the nationality of married women and the debate over protective legislation. Chapter Three highlights the organisation’s gendered approach to peace, including its campaigns for disarmament. Chapter Four investigates WIL’s commitment to internationalism through an analysis of its organisational structure and its work at the transnational level. The final chapter examines how the organisation built and maintained a network of activists, exploring the shared interests between WIL and a range of other voluntary associations, including those working for peace, humanitarian relief, liberal internationalism and socialism. This study firmly places WIL within British and international movements for peace and women’s rights. Work by Leila Rupp, Marie Sandell and Karen Offen demonstrates the wealth of activism by and for women at the international level during the twentieth century. However, previous scholarship has not focussed on WIL in any depth. By offering a detailed analysis of this organisation, the thesis sheds light on a range of issues: the campaign for female citizenship and political participation; the connections between feminism and pacifism; the development of international organisations during the interwar years; and the nature of transnational women’s activism.