Iraq: What might US air strikes achieve?
The formal request from the Iraqi government for the assistance of US air power in seeking to halt the ISIS advance places further pressure on President Barack Obama to act.
This is not the first time the Iraqis have asked for help and up to now Washington has firmly stood back from any engagement.
Additional weaponry has been delivered to the Iraqi forces. US troops and Marines have been sent to reinforce those guarding the US embassy in Baghdad.
But alongside this the Obama administration has been making preparations for potential air operations if required.
A US aircraft carrier - the USS George HW Bush - has been moved into position in the Gulf.
It is accompanied by a number of other warships, not least the cruiser USS Philippines Sea and the destroyer USS Truxton - both capable of launching cruise missiles against targets on land.
If necessary, US air power could be bolstered by other warplanes and support aircraft - like tankers - already based in the region.
Alongside these visible preparations, the Americans will also have been trying to develop as clear an intelligence picture as they can of what is actually happening on the ground.
However, intervening from the air raises all sorts of problems:
- Hitting the target
If ISIS forces are rapidly moving on Baghdad, then US air action becomes more likely. But ISIS is a fast-moving force, mounted largely in light vehicles.
In some circumstances distinguishing them from civilians on the move may be difficult. Might the Americans put some special forces teams on the ground to work with the Iraqi military to help designate and identify targets ?
- How widely to strike?
ISIS is - according to its own documents - a peculiarly well-organised grouping with a clear structure. How much do the Americans know about it? Might they seek to attack its leadership - perhaps by the use of drones?
Geographically, would attacks be restricted to Iraqi territory or would the Pentagon be allowed to hit ISIS-linked targets inside Syria? ISIS has in effect made the border between the two countries irrelevant.
- What of the political context?
Iraqi Prime Minister Nouri Maliki is seen in Washington as in many ways being the author of his own misfortunes - having pursued a corrupt and sectarian approach to government.
The US wants a more inclusive pattern of politics in Iraq. Some Sunnis - true - are supporting the government. But the ISIS onslaught is also indicative of the broader malaise in the country and depends for its success on appealing to the wider Sunni population's sense of grievance.
A US air offensive that appears to be propping up Mr Maliki is hardly going to go down well.
For all these reasons, President Obama's inclination may be to hold fire.
If Mr Maliki can halt the ISIS assault without the intervention of US air power, so much the better.
But if the Iraqi army fails to stabilise the situation and unless ISIS overreaches itself, then the White House may feel compelled to order US warplanes into action.