When Joschka Fischer met Colin Powell in Washington recently, the US Secretary of State gushed with praise for his German counterpart. "He is a true friend of America," the Vietnam veteran and former Gulf War commander declared.

For a politician who spent much of his adult life protesting against US foreign policy, it was an unusual tribute. It reinforced the view of many of Fischer's critics that the German Foreign Minister is a shameless opportunist who has abandoned the principles which once drove his political career.

Fischer's political transformation has been under the spotlight in Germany in recent months as he has sought to explain his role in violent street protests in the 1970s. Prosecutors this week dropped an inquiry into whether he committed perjury during the recent trial of a former terrorist. Most Germans outside the conservative press appear to accept Powell's verdict on the controversy that "what is past is past".

When Fischer visits Dublin on Monday for a debate with the Minister for Foreign Affairs, Brian Cowen, the focus will be less on the German minister's past than on his vision for Europe's future. More than any other European politician, Fischer has set the agenda for the current debate on the future of European integration. His blueprint for a more democratically accountable, federal Europe with a written constitution and an elected president looks set to dominate the negotiations leading up to the EU's next treaty-making summit in 2004.

Slender, silver-haired and always elegantly dressed in conservative suits, today's Fischer cuts a very different figure from the youthful revolutionary of the 1970s. The minister's decision in January to appear as a character witness in the murder trial of Hans-Joachim Klein, a former associate of Carlos the Jackal, started a flood of revelations and accusations about the most turbulent periods in Germany's post-war history.

Although Klein says he regarded Fischer as a role model, the two men adopted very different approaches to revolutionary activity. Like his friend Daniel Cohn-Bendit, now a Green MEP, Fischer opposed the violent campaigns of the Baader-Meinhof group and its successor, the Red Army Faction.

In the highly charged atmosphere of the early 1970s, however, when the Bonn government launched a heavy-handed crackdown on left-wing radicals, street protests frequently turned ugly. A series of photographs released mischievously by Ulrike Meinhof's daughter, Bettina Roehl, show the young Fischer, wearing a crash helmet for protection, attacking a police officer.

Fischer has denied a claim that he encouraged demonstrators to throw Molotov cocktails at police in a riot that followed Meinhof's death in 1976, but he has admitted taking part in violent protests and expressed regret for the suffering they caused.

Born in the southern German state of Baden-Wuerttemberg in 1948, Joschka Fischer was the son of a Hungarian-German butcher who was forced to leave Budapest in 1946. He left school at 16 and started an apprenticeship as a photographer before leaving home to hitch-hike around Europe. At the age of 19, he married his first wife, Edeltraud, in Gretna Green.

By 1968 Fischer was in Frankfurt, earning his living through odd jobs. His main preoccupation, though, was with political thought and he attended lectures at Frankfurt University by Theodor Adorno and Juergen Habermas. Habermas remains an important influence on Fischer to this day.

Working in factories and later as a taxi-driver, Fischer joined a succession of leftwing groups and campaigns before becoming a member of the newly founded Greens in 1982. By the following year, he was married for a second time with two children and when the Greens won 5.6 per cent of the national vote in the 1983 election, Fischer became a member of the Bundestag. He attracted attention the moment he entered the debating chamber wearing a T-shirt, jeans and sneakers. He was 35 years old and weighed 75 kg.

BY 1996, Fischer was the undisputed star of his party and the guiding force behind the electoral strategy that would lead to the formation of the first-ever federal coalition between Social Democrats and Greens a year later. His third marriage had broken up and, after years of eating and drinking too much and exercising too little, he weighed a massive 112 kg. In his book Mein langer Lauf zu mir selbst (My Long Run to Myself), Fischer describes how he gave up drinking and took up running. Within a year, he had lost 36 kg and discovered a fresh sense of calm and purpose. Two years ago, he got married for the fourth time, to Nicola Leske, a journalist 22 years his junior.

While a few Greens were critical of Fischer's abrupt change of sartorial style when he became foreign minister, many more were outraged by the policies he adopted during his first month in office. Promising continuity, he declared that there would be "no Green foreign policy, only German foreign policy".

Fischer's unequivocal support for NATO's military campaign against Yugoslavia split the pacifist Greens, although his most important diplomatic contribution at that time may have been in facilitating a dialogue with Russia and bringing the action back under UN auspices.

When he emerged last year as the most eloquent advocate of closer European integration, some observers suggested that he was following in the footsteps of the former chancellor, Helmut Kohl. Although both men believe Germany's destiny lies in an integrated Europe, their approaches are fundamentally different.

Kohl's European vision was based on fear, above all the fear that resurgent nationalism would once again tear the continent apart. He favoured swift action to bind European nations together. His conviction was so profound that he boasted privately that all the major steps towards European integration had been effected against the will of the German people. In a speech at Berlin's Humboldt University last year, Fischer suggested that if the EU was to move closer to the people of Europe, it must be more democratically accountable. He called for the full parliamentarisation of the Union to create an effective, accountable federation of states.

"That means nothing less than a European parliament and a European government which really do exercise legislative and executive power within the federation. This federation will have to be based on a constituent treaty," he said.

Fischer's most controversial proposal is for a group of EU member-states to be allowed to push forward towards closer integration without the approval of all the others. This principle of "enhanced co-operation" has been incorporated in the Nice Treaty. However, he insisted last month that there should be no question of putting pressure on member-states to move more quickly than they wish.

"Everyone can go as far as they want to. Nobody is forced to join the euro or Schengen or to be in the EU at all. These are free decisions of the peoples," he said.

Although he acknowledges that few Europeans feel much affection for the EU institutions, Fischer maintains that most of us are more European than we realise.

"Very few people here or in France realise how many jobs, how much prosperity, how much social security, how much peace and security we owe concretely to this Europe that appears to them as a bureaucratic giant and which they don't especially want much to do with. The advantages always simply outweigh the rest."