By Jonathan Marcus BBC Defence Correspondent.

The details of how any US attack on Iraq might unfold are still far from certain.

But what is clear is that any assault would almost certainly be preceded by a lengthy bombing campaign to destroy Iraqi air defences and to weaken its forces in the field - especially the armoured formations of the Republican Guard.

In such a campaign Turkish air bases would play a vital role both because of their strategic location and because Turkey, as a Nato member, has compatible military infrastructure from which US aircraft could operate.

The base at Incirlik is the only established US Air Force forward-operating base in the region.

Gulf War precedent

It is located in south-eastern Turkey near the city of Adana and is roughly 500 nautical miles from Baghdad and out of range of any Iraqi ground-to-ground missiles.

Incirlik facts

Houses 500 US and 200 UK personnel Base to F-15 and F-16 fighters, Jaguar reconnaissance aircraft, EA-6B electronic warfare aircraft and KC-135 and VC-10 tankers 3,000m and 2,759m runways

Main source Jane's Defence

It is the base from where the current Northern Watch operation is being run - the operation to maintain an air exclusion or no fly zone over northern Iraq.

During the first Gulf War the US mounted round-the-clock combat operations from Incirlik against targets in northern and central Iraq.

If clearance to fly through Syrian air-space was not available then US aircraft would have to fly eastwards inside Turkey before turning south to head for targets in Iraq.

Other options

Such constraints could make other Turkish bases attractive like Diyarbakir in south-eastern Turkey - some 220 nautical miles east of Incirlik - with a much more direct access route into Iraq.

The regional commercial airport in Erzurum in north-eastern Turkey is another potential fighter base.

There are between 20 and 40 Turkish air bases in all that could be used to support US operations though many have only rudimentary facilities.

But an air campaign involves a huge variety of aircraft.

Most attention is focussed on the combat jets themselves.

But space would also need to be found for Awacs command and control aircraft, tankers, electronic warfare planes and other aircraft.

It is likely that several countries would be involved in furnishing such facilities.

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