[4] In June 1941, well before the outcome of the war was safely predictable, Spinelli and fellow prisoner Ernesto Rossi completed the Ventotene Manifesto, eventually entitled Per un'Europa libera e unita ("For a Free and United Europe.

A Draft Manifesto"), which argued that, if the fight against the fascist powers were successful, it would be in vain if it merely led to the re-establishment of the old European system of sovereign nation-states in shifting alliances.

Because of a need for secrecy and a lack of proper materials at the time, the Manifesto was written on cigarette papers, concealed in the false bottom of a tin box and smuggled to the mainland by Ursula Hirschmann.

It was then circulated through the Italian Resistance, and was later adopted as the programme of the European Federalist Movement (MFE), which Spinelli, Colorni and some 20 others established, as soon as they were able to leave their internment camp.

As in many European left-wing political circles, this sort of move towards federalist ideas was argued as a reaction to the destructive excesses of nationalism.

"[citation needed] The Manifesto criticised the "capitalist imperialism which our own generation has seen expand to the point of forming totalitarian states and to the unleashing of world wars".

[7] After the war, Spinelli, leading the federalist MFE, played a vanguard role in the early episodes of European integration, criticising the small-steps approach and the dominance of intergovernmentalism, feeling even that the chance to unite Europe had been missed as sovereign states were re-established without any common bond other than the functionalist OEEC and the largely symbolic Council of Europe.

It was Spinelli who persuaded Italian Prime Minister Alcide De Gasperi to insist in the negotiation of the European Defence Community (EDC) treaty on a provision for a parliamentary assembly to draw up plans for placing the EDC, the ECSC and any other development within a global constitutional framework to "replace the present provisional organization" with "a subsequent federal or confederal structure based on the principle of the separation of powers and having, in particular, a two-chamber system of representation".

He praised the French communists for providing a "positive contribution to this battle"[12] and said that under his influence the PCI had "adopted the line which I had sought and supported for many years, especially the need to transcend economic unification and move towards a European political union".

[13] The idea was that the European Parliament should act as a constituent assembly, although Spinelli was prepared to make compromises on the way to secure broad majorities behind the process.

This happened with the help of several national parliaments, which adopted resolutions approving the draft Treaty, and of French President François Mitterrand who, following a meeting with Spinelli, came to the European Parliament to speak in favour of its approach, thereby reversing France's policy (since Charles De Gaulle) of hostility to anything but an intergovernmental approach to Europe.

He married Ursula Hirschmann, a German anti-fascist activist and fellow advocate of European federalism, in 1945 and they had three daughters: Diana, Barbara, and Sara.

Prominent supporters of the group are: Jacques Delors, Joschka Fischer, Daniel Cohn-Bendit, Andrew Duff, Elmar Brok.