Zalmay Khalilzad, US ambassador in Kabul since November 2003, has played a hugely influential role in Afghanistan's transition process.
Mr Khalilzad may find things tougher in Iraq
But now the Afghan-American is now expected to replace John Negroponte, as US ambassador to Iraq.
Mr Negroponte has already left Baghdad, but it is not clear when Mr Khalilzad is due to arrive from Kabul.
Perhaps no US ambassador in recent times has been so powerful a player in the country in which they serve as Zalmay Khalilzad.
So much so, that he has sometimes been dubbed the viceroy, or the real president of Afghanistan.
It is unlikely he will manage to exert the same level of influence in Iraq - although he is no stranger to the country.
No major decisions by the Afghan government have been made without his involvement
And Afghanistan has left him well versed in negotiating tribal and ethnic divisions - something he will also need in post-Saddam Hussein Iraq.
With the extent of US involvement in Afghanistan, Washington's ambassador is inevitably a powerful figure.
But Mr Khalilzad's influence has been much greater because of his Afghan birth and long involvement with the country during the years of the Soviet invasion.
Born in northern Afghanistan in 1951, he speaks the country's two main languages - Pashto and Dari.
Mr Khalilzad had particular influence in Afghanistan
After studying in Lebanon, he and his family moved to the US in the 1970s.
He became a US citizen and a Washington insider, with close ties to the Republican party.
He is a protege of Vice President Dick Cheney and Deputy Defence Secretary Paul Wolfowitz.
In Afghanistan, he was not only the ambassador, but also President Bush's personal envoy - but an envoy totally at home with the wheeling and dealing of Afghan politics.
'Here to help'
Inside the heavily-fortified US embassy compound in central Kabul, he lived in two converted containers.
But he had a tent built over the small garden outside, where he could meet key leaders and tribal chiefs in a more traditional style.
President Karzai (left) may now be freer to act
But during his 18-month stay, he was also accused of frequently overshadowing President Hamid Karzai.
It was often he rather than Mr Karzai who announced key initiatives, such as a recent plan to offer an amnesty to members of the Taleban.
To such criticism, he would often respond with disarming charm: "I'm only here to help."
But no major decisions by the Afghan government have been made without his involvement.
When he is in Afghanistan, he sees or speaks to President Karzai several times a day.
The two have known each other for many years and have an easy, informal relationship.
When a temporary crisis erupted during last year's presidential elections - when most of the candidates threatened to boycott the process because of allegations of irregularities - it was Mr Khalilzad who was called in.
On the day, he was seen shuttling between the various camps - his heavily-armed bodyguards close on his heel - until a deal had been agreed.
There have been rumours for some time that Mr Khalilzad was going, although some thought he would stay on to oversee the parliamentary elections planned for September.
Afghanistan's chief justice even wrote to President Bush urging him to extend the ambassador's stay, at least until the autumn vote, sparking a row with the opposition who said he had acted unconstitutionally.
Iraq will not be entirely new to him - he served as President Bush's envoy to the Iraqi opposition in the run-up to the 2003 invasion.
Whoever replaces him in Kabul will struggle to achieve the same level of influence.
But there are some here who say Ambassador Khalilzad's departure is a good thing right now.
As much as President Karzai relied on Mr Khalilzad's support and advice, some analysts argue that it will make it much easier for him to exert his authority, free from the shadow of the 'viceroy'.