What will victory over Islamic State look like? How will we know when the terrorists are finally defeated?

"At the most basic level, you kill or capture the senior leadership," says Anthony Bubalo, a former intelligence analyst and one of Australia's leading observers of Islamist politics in the Middle East.

"You destroy or disperse most of Islamic State's fighting capability. They no longer control any territory. They are no longer able to launch serious military strikes."

It is a set of goals that sounds seductively simple to achieve, especially given the preponderant might of sophisticated Western military force.

But despite French President Francois Hollande's promise of a "pitiless" response to the carnage in Paris, the deadly presence of Russian jets streaking above Syria, or US special forces directing Kurdish Peshmerga in Iraq, the defeat of Islamic State seems a distant hope.

Then came brazen attack on Friday that left almost two dozen dead in a Mali hotel. Although not directly claimed by Islamic State, the atrocity in Mali again illustrated how an extremist danger can linger in the shadows, only to unexpectedly lash out and strike. Two years ago, France launched a major military intervention in the African nation to thwart an Islamist takeover – and last year Hollande declared "the bulk of the work has been accomplished".

Foreign troops cannot win a victory against Islamic State alone, a point Prime Minister Malcolm Turnbull was eager to emphasise several times last week. After a meeting with US counterpart Barack Obama, Turnbull made the difficulties plain:

"He said he could send 50,000 marines into Syria and they would be able to retake Raqqa, and Mosul in Iraq, of course, and they could achieve that success, but what happens after that," Turnbull said.

"When will they come home?"

But Turnbull was also clear no one is willing to brook a compromise with Islamic State. But the problem is where to stop. The military campaign is just one battlefield, and like al-Qaeda before it, Islamic State has won adherents with little or no direct connection.

"Islamic State as an idea, as a movement, it is very difficult to conceive a situation of what victory looks like," says Bubalo, now with the Lowy Institute for International Policy in Sydney.

Social media and online recruiting has transformed into a potent and enduring tool for extremists.

"There is not much you can do, at least initially, about people who self-adhere to Islamic State's ideology, living in Western countries or outside," Bubalo says.

Just over a year ago, Obama was startlingly frank about the challenge of defeating Islamic State.

"We don't have a strategy yet," he conceded.

Most agree the first step must be to deny Islamic State control over its self-professed caliphate overlapping Syria and Iraq.

But Obama is lambasted at home by rivals who complain there is still no plan.

Republican candidate Donald Trump last month wistfully recalled the days of Saddam Hussein in Iraq and Muammar Gaddafi in Libya as more stable times, suggesting Syria would be safer with Bashar al-Assad in power.

The West is now contemplating what once appeared unthinkable, a compromise with Assad in an attempt to unite the opposition fighters in common cause against Islamic State.

"We have no credible option to fill the political space that may be cleared of Islamic State that does not derive in large measure from the role of the Syrian regime," says Bob Bowker, a former Australian ambassador and 40-year veteran observer of the Middle East.

Turnbull last week described Assad as "a murderous tyrant" but said the approach has to be "a pragmatic one".

Without Assad, who will fill the void – let alone do the fighting?

There was much talk in recent months of an "Arab coalition" to fight Islamic State, only for Saudi Arabia to swing its military attention south, to the collapsing state of Yemen.

The small Gulf States have not flown sorties over Syria since March, while Jordan last targeted Islamic State in August, according to media reports.

Vladimir Putin complicated an already crowded battlefield – which also includes Australian fighter jets – by dispatching Russian forces to prop up the Assad regime.

The difficulty is compounded because Kurdish forces will regularly only fight on lands traditionally their own, sometimes dodging volleys from neighbouring Turkey, which fears Kurdish independence.

Iraqi forces have struggled to roll back Islamic State gains, while Syria has become a den of great power rivalry, of regional mistrust and fundamentalist exhortation. All these competing interests will need to be satisfied or eliminated for a lasting solution.

Former prime minister Tony Abbott last week joined a growing chorus of calls for more special forces to be deployed, to direct and support local fighters.

Douglas Ollivant​ served as a US Army officer in Iraq and Afghanistan, and says while special forces usually help, any deployment must be balanced against local political sensitivity.

There is also the balance of risks to Western forces themselves.

"To bring up the obvious catastrophic one," Ollivant tells Fairfax Media from Washington, "what if one of these special forces guys ends up being burned alive on national TV. What will that do to the public will for the war effort?"

Ollivant returned last week from a 10-day trip to Iraq, meeting local officials and seeing first-hand life in the country. He is realistic about the prospects of defeating Islamic State.

"I don't think anyone is under the illusion every [Islamic State] fighter in Iraq, for example, is going to be killed. Some of them are going to melt back into the woodwork, and go back to being a terrorist organisation in the shadows."

But despite the impression of chaos in Iraq, Ollivant said he was struck by the surprising normality of life.

It points to the risk of overestimating the strength of Islamic State and accepting uncritically its bombastic claims of sweeping success. The attacks in Paris can also be interpreted as a sign of weakness, the Islamists lashing out as they are pressured in former strongholds.

Yet even should Islamic State be "degraded", the problems in the Arab world could get worse.

"There is almost no Arab government at the moment, or in prospect, that actually gives people a sense of comfort in the direction in which those Arab governments wish to take their countries," says Bowker.

The collapse in Syria, Libya and Yemen, the fragility of Egypt, and low oil prices raising the prospect of financial crisis in Saudi Arabia and other oil-producing nations is a deep concern.

"I think we have to invest in the Arab world, not only terms of economic engagement, but also we need to be welcoming its refugees, we need to be fostering ideas of a more inclusive empowered and empowering vision of an Arab future," says Bowker.

He warns treating the Middle East as a threat will be a self-fulfilling prophecy.

"There is a demographic and financial time bomb ticking in the region, which will shape the Arab future by combining with the fragility of government institutions and the shortcomings of Arab leadership in this generation," Bowker says.

Otherwise the extremists fill the void with hate. Knowing victory is hard, failure will be all too obvious.