Illusions of common interest and joint purpose falter when states choose to break up, as with the recent changes to the European Union, or according to more dangerous precipitants such as those which shaped the Franco-German Armistice 1940, 80 years ago as a detail of war. The latter bares the sudden end of the Franco-British alliance and holds an invitation from history to re-examine the troubling political, social and legal layers of the concept of the vital interests of states. That category opened to radically different interpretations for political and legal thinkers who witnessed the fall of France yet did not respond directly or immediately. Hannah Arendt’s theory of politics, conceived in the aftermath of war as a corrective to the internal fragmentation of the European nation-state, elucidates the instability of the concept of vital interests which underpinned international legal and political thought in the 1930s and 1940s and frustrates the co-operative relations between states. The problem pairs back, she says, to whether interests signify an associative technique or sword. Her invitation for legal thought is to challenge the expectation of rupture implicit in the juridical category by outlining an alternative that recovers the pacifistic function of law and implicates the international lawyer.