There is an appealing normality about Jack Straw. No sooner had the Foreign Secretary negotiated a crucial deal in Tehran to try to defuse the Iranian nuclear crisis than he returned to his Airbus and changed into his pyjamas before dinner with senior officials. He didn't sleep, though. Shuffling to the back of the plane in his socks to chat to journalists, he smiled but was still playing back the day's events in his mind.
"It's a bit like the feeling you have after final exams," he explained. "I feel good about it. But there's a bit of Protestant self-flagellation." This caution could not be more different from the dashing presence of the French poet-diplomat Dominique de Villepin, who left Tehran declaring that "it's an important day for Europe".
But Straw had reason to be pleased. Travelling to Tehran as part of the new European triumvirate - comprising the foreign ministers of Britain, France and Germany - he had helped to secure what may be one of the most important diplomatic agreements of his two years as Foreign Secretary. He is seeking a grand bargain that will neutralise Iran's nuclear programme, stay America's sword, reunite Europe and bring Russia alongside with the promise of nuclear contracts with Tehran. If all this succeeds - a big if - the Europeans may just be able to draw Iran back from "going nuclear" and setting off a regional nuclear arms race, if not provoking a pre-emptive war by America or Israel.
The trio could demonstrate that European diplomacy - not just American military might - can deal with the threat of weapons of mass destruction. The International Atomic Energy Agency has given Iran until the end of the month to prove that it is not trying to build a nuclear bomb. If it fails to comply, it faces being referred to the UN Security Council for possible sanctions. Iran says it only wants nuclear power to generate electricity, and has in practice agreed to all the agency's demands. These include "temporarily" suspending work on enriching uranium.
If Iran masters the art of making the enriched uranium needed for civil reactors, it could secretly make the highly enriched uranium necessary for nuclear bombs. The trouble is that uranium enrichment is ostensibly legal under the Nuclear Non-Proliferation Treaty. To get round the problem, the Europeans have offered an inducement: if Iran scraps the enrichment plant being built at Natanz (as well any attempt to reprocess spent fuel rods to extract fissile material for bombs), Europe will guarantee to supply nuclear fuel even if Russia doesn't deliver.
Straw knows that Iran has made a tactical move under pressure. But as one senior aide said: "Sometimes a series of tactical steps can become a strategic change."
The three foreign ministers may have stumbled on a new model for European common action: the "bigs" set the pace, the "smalls" follow suit and, as a result, the EU has a chance of setting the agenda for Russia and America. By travelling together, the ministers not only presented a united front to Iran but found safety in numbers in dealing with Washington. The message to the Iranians was: "Deal with us or face the American army on your doorstep in Iraq." But the unspoken message to the Bush administration was: "Don't try to drag us into another war."
Yet there are echoes of the Iraq crisis: Iran's evasions on the nature of its weapons programme; problems over inspectors; attempts to split the international community with concessions; and an ultimatum for compliance.
The wars in Afghanistan and Iraq have turned the US and Britain into Iran's intimate neighbour. Deployed to the east and west, the allies can exert more pressure on the mullahs, but they are also more vulnerable to Iranian threats. The situation in Iraq is bad enough with part of the Sunni population in revolt. But the occupiers' position would be all but untenable should the Shia majority rise up as well. To keep them onside, it is best to avoid a bust-up with the Shia clerics who run Iran.
Straw has staked much of his credibility on bringing Iran in from the cold. Tehran has become one of his favourite destinations, with five trips under his belt. This was partly a necessity of the "war on terrorism", but also a choice by Straw to try to turn the regime from a pariah into a friend. In Iran, Straw believes Britain can make a difference, and in turn show that Britain is different from America. Hawks in the Bush administration are already making clear their displeasure.
If the strategy of engagement fails, Straw will be accused of being dangerously naive in wooing a member of the "axis of evil". US hawks argue that Iran must not be allowed to develop any kind of nuclear facilities. They say that even if Iran accepts all safeguards demanded by Europe, there remains the danger that Tehran will one day emulate North Korea by seizing the nuclear fuel in its reactors and reprocessing enough to make dozens of weapons.
For many in America, Iran is a menace that must be eliminated, not rehabilitated - particularly not at a time when, they hope, popular disaffection with the mullahs could sweep the regime away. As Straw was whisked from one former palace to another in Sa'ad Abad - one of the former shah's royal compounds on the slopes of the Elburz Mountains - he appears to have taken a different view: the Islamic revolution is a reality, but the west can help it evolve into a more friendly form.
Iran may yet determine Straw's place in the annals of British diplomacy. "If the only thing we do in life is solve the Iran nuclear problem, then we would have achieved something," he said.
Anton La Guardia is diplomatic editor of the Daily Telegraph