In April 1940, during the Second World War, the French philosopher and thinker Jacques Maritain advocates the federal reconstruction of post-war Europe, seeing in the federalist approach a solution to the problem of Germany.
In December 1942, Altiero Spinelli and Ernesto Rossi address a secret letter to Count Sforza, then in exile in the United States, on behalf of the Italian federalists kept under house arrest on the island of Ventotene, enjoining him to support the post-war realisation of the United States of Europe.
Le 29 janvier 1943, l'écrivain allemand Thomas Mann prononce à New York un discours radiodiffusé dans lequel il dénonce le nouvel ordre européen prôné par les nazis et défend l'idée d'une Europe fédérale libre.
In November 1943, the Italian Movement for European Federation calls upon all francophone anti-Fascists to work together for the creation of a United States of Europe, freed from the yoke of totalitarian regimes.
On 25 March 1944, the Legal Affairs Committee of the Pan-European Movement and the Research Seminar for Post-War European Federation adopt in New York a draft federal-type constitution for a United Europe.
On 31 March, 29 April, 20 May and 6 and 7 July 1944, militants from resistance movements of several European countries meet secretly in Geneva to discuss the problems related to the reconstruction of a democratic, federally-based Europe after the war.
In May 1944, the Italian underground periodical L’Unità europea, organ of the European Federalist Movement, highlights the efforts undertaken by European Federalists against Nazi Germany and Fascist Italy to achieve European unity.
In June 1944, members of the French Resistance, meeting as the French Committee for European Federation, deliver a statement in which they show their support for European federalism and define the shape of a united post-war Europe.
Head of the French Section of the Workers’ International (SFIO), Léon Blum led the Government of the Popular Front from 1936 to 1937. Imprisoned by the Vichy authorities, he was later deported to Germany where he pursued his internationalist ideal.
In 1945, Frédéric-Joseph Vandemeulebroek, Mayor of Brussels, welcomes Winston Churchill, former British Prime Minister, in order to confer upon him the freedom of the city of Brussels for his heroic actions during the Second World War.