He began a lifetime involvement with revolutionary politics in the late 1930s in Greece. Drawn into the "Spartacus" faction, he represented Greek Trotskyists at the founding conference of the Fourth International in Paris in 1938. Born in Alexandria, Egypt, Pablo graduated from the Athens Polytechnic and continued his studies in urban planning at the Sorbonne in Paris, where he was to spend much of the following decades. During the 1936 military dictatorship of Ioannis Metaxas Pablo was arrested and exiled in the Aegean island of Folegandros. There he was not admitted by the orthodox communists, also in exile, so he joined the company of cattle and horse thieves, who at that time were punished with exile. In Folegandros he met his future wife Elli Dyovounioti. Together they escaped from the island and later from Greece. Pablo was ill in Paris when the Second World War began. The same ill health meant until 1944 he played little part in the activities of the French Trotskyists although he was reported to have given educational classes to David Korner's Union Communiste. By 1944 he was fully involved with the movement, and was elected the organizational secretary of its European Bureau, which had re-established contact with between the Trotskyist parties.
After the war, Pablo became the central leader of the Fourth International with the support of the SWP of America and James P. Cannon. Pablo played a key role in re-unifying, re-centralising and re-orienting the International. In 1946 Pablo visited Greece to successfully reunify the four separate Trotskyist parties. Pablo and Ernest Mandel were instrumental in these years in winning the Fourth International to a position that asserted that the Eastern European states conquered by the Soviet Armed Forces in 1944-45 had by 1948 become what they described as deformed workers' states.
In the uncertain aftermath of the second world war, when the Trotskyists were numerically dwarfed by the mass Communist Parties and their hopes for a revolutionary breakthrough were dashed, Pablo also advanced a new tactic for the FI from about 1951 onwards. He argued that a Third World War, which was believed by many people to be imminent, would be characterised by revolutionary outbreaks during the actual war. Splits of revolutionary dissenters were likely to develop in the Communist Parties. To gain influence, win members and avoid becoming small sectarian cliques just talking to each other, the Trotskyists should - where possible - join, or in Trotskyist terminology enter, the mass Communist or Social Democratic (Labour) parties. This was known as entrism sui generis or long-term entry. It was understood by all that the FI would retain its political identity, and its own press.
It was believed at the time that the international "centre" should be able to impose democratic centralist discipline by directly intervening in the politics of local parties. Pablo also used the weight of the international secretariat to back tendencies that were closest to mainstream views inside the International. For example, Pablo and Cannon jointly sponsored an entryist faction within the British movement that opposed the leadership of Jock Haston in the RCP, contributing to the collapse of the RCP as an open organisation.
In 1953 the American, British and part of the French Trotskyists declared themselves in opposition to this course of action, and withdrew from the FI to form a public faction, the International Committee of the Fourth International.
The subsequent hostility of the ICFI to what became known as "Pabloism" became legendary. Pablo was demonised, and made into the exemplar for everything that had been criticised in the Trotskyist movement. Decades later, a few small Trotskyist sects are still writing "anti-Pabloist" tracts.
Pablo himself continued with the European International Secretariat of the Fourth International, operating from Amsterdam and Paris. In reality, though, the entryist tactic he proposed could not be implemented in many countries and succeeded only to some extent in countries where a large social-democratic party could be 'entered'.
None of the various Trotskyist splinter groups gained large numbers of new members in the early Cold War years, whether 'independent party-builders' or 'entryists'. After the invasion of Hungary in 1956, many intellectuals split from the Communist Parties, and there was further political fragmentation resulting from the Sino-Soviet split, but the Trotskyists gained almost no new adherents from them.
As the 1950s became the 1960s, Pablo was convinced that the best revolutionary prospects were now in what was to become known as the Third World of Africa, Latin America and Asia. He also wrote a prophetic essay anticipating the women's liberation movement.
He was personally closely involved in supporting the Algerian national liberation struggle against France, which led to imprisonment in Holland in connection with counterfeit money and gun-smuggling activities. A campaign for his release was launched by Jean-Paul Sartre. In 1961 Pablo was finally sentenced to 15 months imprisonment, and liberated at the end of his trial. He took refuge in Morocco. After the victory of the Algerian revolution, he became a minister in the FLN government.
By 1963, inspired by common positions towards the Cuban revolution, the ICFI forces around the SWP of the USA were moving back towards unity with the ISFI. Pablo was regarded by the SWP as a barrier to that unification. The world congress in 1963 formed the reunified Fourth International. Pablo moved a counter-resolution at the 1963 reunification congress, as well as the main resolution on Algeria, and was elected to the international executive committee. Tensions grew, and Pablo and his African Bureau were outside the International by the end of 1965 for partly disputed reasons: in the view of Pablo's supporters, reunification rapidly led the new majority to oust Pablo; in the International's view, Pablo's tendency broke with the International publicly and placed itself outside the FI. What is not disputed is that by then Pablo had key political differences with the FI.
Pablo's influence through this period then was mainly through his writings. The central theme of Pablo's thought in the later 1960s and 1970s was that of autogestion or "workers' self-management" (Arbeiter-selbstverwaltung). At the beginning of the 1970s, he was politically active in Chile, under Allende's government. After the fall of the Junta, he returned to Greece. In the 1980s, however Pablo receded into political obscurity.
Pablo continued with his revolutionary politics, and organized the Revolutionary Marxist Tendency and the International Revolutionary Marxist Tendency based in France. These were never serious competitors to the larger Trotskyist groups. The sections of the IRMT rejoined the reunified Fourth International in 1994 and 1995, although the agreement was not applied in Pablo's individual case.
Unusually for a revolutionary, his funeral was a state event in his native Greece. This is explained by his personal friendship from the 1930s with Andreas Papandreou who had been a Trotskyist in his youth. Pablo's motto was: "The meaning of life is life itself, to live as much as you can".