The purpose of the horrific video of the beheading of US photojournalist James Foley seems straightforward – to terrify the US into halting its aerial bombardment of the militant group.

But what appears to be a barbaric and blood-soaked ransom note to force a US withdrawal has an entirely different objective, say former national security operatives and terrorism experts.

That is, to generate enough public outrage so the US and its allies expand their military campaign in Iraq and Syria to include ground forces.

Released by terrorist group Islamic State on Wednesday, it begins with footage of US President Barack Obama announcing air strikes on Iraq before Foley, on his knees in the desert and reciting scripted remarks, blames the US government for his impending death.