David Kilcullen, an Australian army veteran, became the senior counter-insurgency advisor to US General David Petraeus during the Iraq War and is considered a key architect of the "surge" strategy that helped turn the conflict around.
In an exclusive interview, he told AFP the Paris attacks showed the Islamic State group (IS) was morphing from a terrorist threat into a "structured organisation" like the IRA in Ireland or ETA in Spain during the 20th century.
"This is not like the expeditionary terrorist attacks of Al-Qaeda where they build a team in one country, smuggle them into another country and they all die," said Kilcullen, whose book "Blood Year: Islamic State and the Failures of the War on Terror" is due out next year.
"The way the Paris attacks went down with safe houses, stolen cars, weapon caches, guys successfully going underground after attacks -- this is a lot closer to the classical definition of urban guerrilla warfare."
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"We need to have an honest conversation -- we're pretending we're not in combat, that we don't have boots on the ground," he said.
"But we have several thousand on the ground in Iraq and the 50 that are acknowledged in Syria are not the only Western special forces there.
"We're trying to present this fiction that the war in Iraq is over."
Kilcullen rejected suggestions from some analysts and politicians that the West should deploy tens of thousands of troops.
"And frankly, it wouldn't work. The Iraqis and Iranians would push back hard.
"And a small-scale ground invasion is asking for trouble because you put enough guys in to really piss everyone off, but not enough to win."
What is really needed, said Kilcullen, is "a serious air campaign".