President Bush's surprise visit to Iraq today was notable for its timing, and for its symbolism.

It came as political Washington returns from its summer recess and prepares for a crucial debate on the future of the US intervention in Iraq.

Early next week General David Petraeus, the top US commander in Iraq, and Ryan Crocker, the US ambassador to Baghdad, will testify before both the US House of Representatives and the Senate, and by the end of the week the White House will send Congress its own progress report on the situation in Iraq. Mr Bush wants to sustain the present 'surge' of an additional 30,000 US troops in Iraq well into next year, while many of Congress's ruling Democrats are demanding a timetable for withdrawal from a conflict that has cost $10 billion a week and the lives of 3,700 American soldiers.

Mr Bush needs to show that the 'surge' is working, which is why he chose to land not in Baghdad, but in the remote air base of al-Asad in Anbar province.

Anbar is a rare success story for the US military in Iraq. It is the heart of the once-infamous Sunni Triangle, but in the past few months has been transformed from one of the most dangerous parts of the country - an al-Qaeda stronghold and the hotbed of the anti-US insurgency - to one of the most peaceful.

The military has achieved this by forging an alliance with the province's sheikhs, or tribal chiefs, whose supremacy was challenged by al-Qaeda and who were repelled by al-Qaeda's rule of terror. The sheikhs persuaded thousands of their followers to join the previously-reviled Iraqi police. The US military established combat outposts in some of the province's most dangerous areas. Working together, they have largely driven al-Qaeda from cities like Ramadi and Fallujah.

Mr Bush's visit was also significant for where he did not go - namely Baghdad. While he can boast that the 'surge' of 30,000 extra troops has improved security in Iraq, he has expressed disappointment at the Shia-led Iraqi government's failure to use that breathing space to promote reconciliation with the Sunnis. Nouri al-Maliki, the Iraqi prime minister, instead flew to meet Mr Bush in Anbar province, which was another important piece of symbolism: a Shia leader visiting an overwhelmingly Sunni province.

Mr Maliki has been working hard to deflect criticism of his government in the past two weeks, reaching an agreement with other leaders on measures that include a relaxation of restrictions on former Baathists taking top government jobs, and the release of thousands of Sunni detainees held without charge. Mr Maliki said yesterday that if the Sunni ministers who quit his cabinet did not now return, he would appoint different Sunnis to their jobs next week.

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