Ernest Mandel, who has died in Brussels aged 72, was one of the most creative and independent-minded revolutionary Marxist thinkers of the post-war world.  His writings on political theory, world history and Marxist economics were translated into 30 languages and in every continent.  In a series of specialist works -- Late Capitalism (1975), The Second Slump (1978), The Long Waves of Capitalist Development (revised and re-issued in 1995) -- he analysed the functioning of capitalism in the West.

Mandel had been a prominent leader and theoretician of the Fourth International from the late fifties onwards, but even those on the left who were not sympathetic to his Trotskyist politics acknowledged his influence and demonstrated a respect for his razor-sharp intelligence.  Only a few years ago, Mandel shared a platform in Madrid with the Prime Minister Felipe Gonzales and subjected his host to a severe tongue-lashing for arresting young people who were resisting conscription.

He was born in Belgium and was educated at Brussels University and the Ecole Pratique des Haute Etudes in Paris.  His father Henri, a leftwing socialist, had opposed the first world war and fled from Belgium to Holland to avoid conscription.  Here he met the German communist Wilhelm Pieck, and both men rushed to Germany after the fall of the Kaiser.

Henri Mandel worked in Berlin for several months as a journalist for the newly organised Soviet Press Agency.  He also became a friend of Karl Radek, the Bolshevik emissary despatched by Lenin to speed up the German revolution.

Demoralised by the repression which followed the execution of Rosa Luxemburg and Karl Liebknecht, Henri remained a member of the German Communist Party for only a few more years.  Then he dropped out of active politics and moved to Antwerp.  It was here that his second son, Ernest, was born.

Mandel was 10 when Hitler came to power.  Years later he told me, “My father made some very sharp comments at the time on the incapacity of the social-democrats and the communists to resist fascism.  I remember him saying ‘This will end very badly.  It could be the end for our people’.

In 1939 Mandel joined a small Trotskyist group in Antwerp and was active in the Resistance during the occupation.  He had been disgusted by the capitulation of the Belgian Socialist Party, whose leader, the deputy Prime Minister, made a public appeal to collaborate with the Nazis and was supported by an important section of the trade union apparatus.  The official Communists published a legal paper under the Occupation, basking in the deadly rays of the Stalin-Hitler pact.

Mandel was arrested for the first time for distributing seditious leaflets to the occupying German soldiers.  He had subsequently hidden to observe the effect of anti-fascist propaganda on the uniformed Germans.  He was a revolutionary and a Jew.  The Nazis sent him to a transit camp for prisoners en route to Auschwitz.  He escaped.  The circumstances in which he freed himself are revealing and made a permanent mark that fuelled his optimism about the capacity of ordinary people to emancipate themselves.

Always a strong believer in his own capacity to convince anyone of the merits of socialism, Mandel started talking to the warders.  The other Belgian and French prisoners were anti-German and treated the warders, veteran employees of the German state, as sub-humans.  Mandel started talking to them and discovered that some had been members of the now-banned social-democratic and Communist parties in Germany.  The warders, on their part, were impressed by the precocity of the 16-year-old boy in their charge and actually helped him to escape.

Even though he was soon re-arrested the experience had made him an internationalist.  He steadfastly refused to write off a whole nationality because of the crimes of its leaders.  A lesson learned when our century was engulfed in what seemed then to be a permanent midnight was applied more recently to the war in former Yugoslavia. Mandel refused to permit his loathing of Milosevic and Tudjman to lead to a blanket condemnation of Serbs or Croats.

After the war, Mandel devoted most of his energies to building the Fourth International as a world party for the socialist revolution. He genuinely believed that conditions would favour the re-birth of a movement not tarred with the crimes of Stalinism or the capitulations of social- democracy. During the late sixties and seventies, his polemical and oratorical skills (he spoke all the major languages of his continent) together with governmental paranoia led to his being barred from entering the United States, France, West Germany, Switzerland and Australia. He was deemed a threat to “national security.

The restriction on his movements sent him back to his old typewriter. Pamphlets and books emerged at an amazing speed.  He was a great educator.  His pamphlet, “An Introduction to Marxist Economics,” sold half-a-million copies.  And yet a great deal of his life was spent on dealing with the views of rival Trotskyist groupings.  Often, when I rang him during the seventies, and asked a polite “How are you?” the reply was never the same: “I’m just finishing off a draft reply to the sectarians in Ceylon on the Tamil question” or “Fine. Have you read my reply to the IS Group on state-capitalism?” or “Those sectarian idiots in Argentina have caved in to Peronism. Crazy people. Don’t they understand?”  They never did, but Mandel never stopped trying to convince “crazy people” to tread the true path.

I was very close to him for the most important years of my life.  Even after I left his movement in 1981, we remained close friends.  Friendly relations were abruptly broken off for a year after the appearance of Redemption, my fictional satire on Trotskyism.

The central character, Ezra Einstein, was loosely based on him.  “Tell me something,” he said when peace had finally been restored, “why did you make me out to be so obsessed with sex?”  I tried to explain that a true political Utopian would also be a sexual Utopian and that in fiction one had the right to imagine anything, but he shook his head in disapproval. He was capable of theorising on any subject under the sun, but not sex. This divide with the generation of ‘68 was never overcome.

He suffered a serious heart attack a few years ago, which left him extremely frail, but up to the last he was thinking of new projects.  “I can’t decide what book to write,” he told me last year.  “A history of the European workers movement or the permanent and eternal links between capitalism and crime.”  In the event he wrote neither.

Like many of his generation he was shaken by the restoration in Russia, though he tried to mask the fact with heady rhetoric.  He knew the game was up for another four or five decades, but was fearful of “demoralising the cadre” and a pretence was maintained that nothing fundamental had really happened.  Life and the struggle went on just as before. The truth was that in the changed mood of present times his optimism had ceased to be infectious.