When a city suddenly moves to the top tier of foreign ministers' travel itineraries, you know it has notched up new political significance.
Such is the status of Tehran.

No sooner was a landmark nuclear deal announced in Vienna on 14 July than senior officials started checking and changing their diaries.

The first to reach the Iranian capital was Germany's Vice-Chancellor and Economy Minister Sigmar Gabriel, who flew in with a 100-member delegation, including many captains of German industry and finance.

Now it's the turn of the EU's foreign policy chief Federica Mogherini, who's just arrived for her first official visit. Europe's top diplomat played a key role in the final stages of the intensive talks which stretched over nearly two years, sometimes hovering on collapse.

Now she wants to play a leading role in the implementation, which will also be demanding and difficult, to ensure the agreement holds: that Iran carries out significant curbs to its nuclear programme to cut off all pathways to any possible development of a nuclear bomb - and that world powers reciprocate with the lifting of sanctions.

And that's not the only reason why Iran will now be under the world's microscope. Even Ms Mogherini's itinerary underlines another concern.

Her diplomacy this week began in Riyadh with her first official meeting with Saudi Arabia's Foreign Minister Adel al-Jubeir, who on Monday condemned what he called recent "aggressive statements" by Iran.

That means Ms Mogherini arrives in Tehran fully briefed on the deep-rooted concerns in many Arab capitals over a nuclear deal they fear that will embolden Iran in its regional political ambitions.

 

Before she set out on her trip, Ms Mogherini told the BBC she hoped the deal could be "a great opportunity to open the country".
'Decreasing hostility'

When I travelled to Tehran a year ago with her predecessor Catherine Ashton, no-one could say for sure if a deal would be done, but Tehran felt like a city willing it to happen.

Everywhere we went - from popular bazaars to business offices - Iranians approached us to say how much they hoped this long period of punishing sanctions would soon come to an end. Many expressed hope Iran would re-engage with the West, and take its rightful place at the world's top tables.

But there was also sensitivity among more conservative Iranians that the nuclear deal was a Trojan horse to allow the West to start meddling in Iran's internal affairs, including human rights and issues of personal and political freedoms.

And yet there were, and are, the slow but certain signs of change.

A year ago, European tourists were starting to trickle back in greater numbers. At the sprawling Grand Bazaar with its exquisite traditional mosaics, we ran into a group of delighted German tourists who were gushing in their praise of Iranian hospitality.

Now even countries like Britain, whose embassy has been closed since attacks on the property in 2011, recently relaxed its travel advice for most areas of Iran, saying there had been "decreasing hostility under President [Hassan] Rouhani's government".

A year ago, in bustling hotel lobbies, we met people from Asian and Arab and African countries who spoke of keen interest in a nation with a big population, and big potential. Iran, under sanctions, was already a destination in the diary of many officials and investors.

Now a fast-flowing stream of visitors is swelling with the arrival of more business executives from the West, as well as Russia, China and many other nations hoping to exploit business opportunities and make this new opening to Iran even wider.
Daring to hope
When the EU in Brussels unanimously approved the nuclear deal earlier this month, Ms Mogherini again hailed this diplomatic triumph in resolving, peacefully, a major security problem of our time - without a shot being fired, without it being a zero-sum game.

So great was this sense of achievement that the EU's top diplomat moved to build on this momentum and establish a similar diplomatic forum to deal with an even more protracted problem: the Israeli-Palestinian conflict.

But the challenges for the world's top diplomats on a still controversial deal are only beginning. We'll get an inkling of what lies ahead through official remarks on Tuesday, what we hear on the streets, and through the coverage in Iran's often outspoken media.

Ms Mogherini will hear even more behind closed doors with she meets Iran's top leaders on her one-day trip. And hours after Iranian officials bid her farewell, they'll welcome France's Foreign Minister Laurent Fabius to Tehran.

In September, Austria's president will become the first head of state to visit after a deal clinched in his own fine capital.

There's a real sense here that a page is being turned. But there's still no real certainty about how this new chapter will be written, even if most Iranians dare to hope there's a happier ending in store.

This article was written by Lyse Doucet for BBC on July 28, 2015. Lyse Doucet is Chief international correspondent.