Former CND activist brokers diplomatic breakthrough of the decade as years of dogged on-off negotiations finally pay off

When Catherine Ashton started on the daunting task of building the European Union's first diplomatic machine in late 2009, the Labour peer was met by guffaws of derision.

"Lady Qui?" they sniffed in Paris. In Berlin, they complained that Germany was getting short shrift. Besides, none of her people spoke German. In London, the attitude was "Britain does not want a European foreign policy and she'll never deliver one. So fine."

Amid this general climate of contempt, disappointment, and surprise, a senior EU official who went on to play a central role in her diplomacy offered a dissenting voice: "In four years' time Ashton will be a major figure."

As dawn broke over Geneva on Sunday, that remark from November 2009, almost four years to the day, looked rather prescient. The former Campaign for Nuclear Disarmament activist had brokered what looks like the biggest nuclear de-escalation of an era, the diplomatic breakthrough of the decade, a problem and a dispute so intractable it could have led to a devastating war engulfing the entire Middle East and beyond.

The partial but significant defusing of the Iranian nuclear question is no doubt fundamentally due to the change of regime in Tehran this summer and the Obama administration's decision to get serious about talking to Iran for the first time in a generation.

But Ashton's dogged nurturing of years of on-off negotiations, what is described in Brussels as her "emotional intelligence" in steering and mediating the highly complex talks, paid off handsomely. On Sunday, she found herself in the unaccustomed position of being deluged with compliments.

"I would like to congratulate in particular Catherine Ashton, the high representative/vice-president of the European commission, for this accomplishment, which is a result of her tireless engagement and dedication to the issue over the last four years," said her boss, Jose Manuel Barroso, president of the European commission.

Herman Van Rompuy, who chairs European summits of national leaders, said: "I commend Ashton for her crucial role – as negotiator and co-chair of the talks. Her dedication and perseverance have been key in brokering this first agreement."

John Kerry, the US secretary of state, hugged Ashton tightly and paid tribute to her mediation skills as "a persistent and dogged negotiator". He added: "I'm grateful for her stewardship of the talks."

Van Rompuy and Ashton got their jobs at the same time as a result of the Lisbon treaty, which created the posts of president of the European council and high representative for foreign and security policy.

Both were obscure figures, seemingly quite unsuited to leadership, strategic vision and policy formation. Which was precisely what Europe's main national leaders wanted. They did not want a Tony Blair or a David Miliband or forceful German or French politicians strutting the international stage, setting the policy agenda and outshining them.

What they opted for and what they got were two quiet, methodical, effective fixers and mediators wrestling with some of the biggest issues of the age. It fell to Van Rompuy to deal with quarrelling national leaders over the EU's worst ever crisis – the euro, the sovereign debt and financial turmoil.

Ashton had to build an EU diplomatic service from scratch, creating the EU's first new institution in a decade, amid some of the most vicious infighting within Brussels and between Brussels and the 28 member states.

Much of the criticism levelled at her was veiled sexism and it hurt. She retreated into low-profile workaholism, crisscrossing the globe, avoiding the media, assiduously and slowly building personal rapports with players such as the Iranians, Hillary Clinton and her Chinese counterpart. In the Balkans, she inaugurated highly personalised diplomacy with the Serbian and Kosovo prime ministers that has also produced a little-noticed, but major breakthrough.

A couple of weeks ago, Serbs, who refuse to recognise a breakaway, independent Kosovo, took part in local Kosovo elections for the first time, tacitly if grudgingly coming to terms with the legitimacy of Kosovo government.

It is quite certain that this would not have happened without Ashton's endless engagement and mediation between the two sides through dozens of meetings and late-night dinners.

Describing Ashton's approach to the Iranian negotiations, a former senior EU official said: "You can achieve all sorts of things if you let others take the credit."

The Balkans required a different tack: "Where she deserves enormous credit is on Serbia/Kosovo where her personal contribution cannot be overestimated."

By contrast though, EU foreign policy suffered a big blow last week when President Viktor Yanukovych of Ukraine abruptly ditched a strategic pact with Europe to have been sealed with Ashton at an EU summit in Lithuania this week.

In Geneva at the weekend and a fortnight ago, the format was a dizzying array of "bilaterals", separate meetings between the Iranians and each of the six other countries as well as countless sessions between any two of the six countries. Then there was the odd plenary session with everyone present.

In this complex multi-dimensional diplomacy, the only person almost always present with an overview of everything was Ashton. It fell to her to summarise, cajole, narrow differences, take messages back and forth.

Much of the spadework in earlier negotiations was done by Robert Cooper, the retired British and EU foreign policy strategist and diplomat. These days that role is filled by Helga Schmidt, the German EU diplomat who once headed the office of Joschka Fischer, the former German foreign minister and Greens leader.

The weekend breakthrough is but the first stage, lasting six months, towards a "comprehensive' settlement of the dispute with Iran. Whether that can be achieved in the 11 months that remain to Ashton in her post is arguable. But she can take a large part of the credit already for operating within the limits of the possible and facilitating a deal that defied all sides for more than a decade, since revelations of Iran's clandestine 20-year-old nuclear programme exploded in 2002.

In Europe they are queuing up, mainly men, to replace her next year – Radek Sikorski in Warsaw, Carl Bildt in Stockholm, while at the weekend there was talk of Frans Timmermans, the Dutch foreign minister.

Ashton, then with zero foreign policy experience and a politician who has never held elected office, did not know she was getting the job, becoming the highest paid diplomat in the west, until two days before she was named in 2009.

She was surprised. In Geneva at the weekend, it was her turn to surprise.