We've seen how ordinary citizens around the world have responded to the Charlie Hedbo terrorism, but how will world leaders react? Is marching enough or is it time for troops?

This week well over a million people marched in Paris to defend the values of the French republic. Forty international leaders accompanied them; it was an impressive display of solidarity with values that are deeply held in most western nations.

Whether you think it is wise or not to gratuitously insult a religion, there can be no doubt the ideals of free speech permit this, and it is the right to speak freely without murderous intimidation that is being defended. If terrorists can prevent the exercise of free expression through murder, what other freedoms will be threatened?

So we come to the question of what happens next.

Can the West allow terrorists, homegrown or not, to have safe haven for training and for hatred? For this is now the challenge. Police actions in France and in other countries will not defeat homegrown terrorists, not so long as there is the magnet of a safe haven that is secure from western efforts to defeat them.

This is the effect of the huge swath of territory now controlled by ISIS. It is a terror state, or at least a proto-state. And is the magnet for hundreds of disaffected people from western nations who can now go to ISIS and be trained, armed and prepared to attack their home countries. We know that at least five New Zealanders have joined ISIS and apparently up to 200 Australians have done so. So long as ISIS exists, it will draw in jihadists who could all too easily pose a threat to their home countries. The existence of ISIS has also goaded Al Qaeda to try to outdo ISIS. In effect, if not intent, they are evil twins.

Of course removing ISIS is nowhere near a complete answer to defeating terrorism, but right now it is an essential element in curbing the scope of the terrorist scourge. So long as ISIS exists there will be a level of terrorist events that is much greater than it would otherwise be.

Is the current air campaign lead by the United States and France going to be enough to remove ISIS? It does not look likely in a credible timeframe. Once ISIS fighters learned to stay close to the towns and cities, they made the air campaign much less effective. The border with Turkey seems so porous that pretty much anyone who wants to enter ISIS can do so. This is why General Martin Dempsey, Chair of the Joint Chiefs, indicated that a ground campaign could be necessary to defeat ISIS.

So western leaders have some hard choices. They can continue with the existing air campaign. It will no doubt hurt ISIS, but it will not defeat ISIS. And not defeating ISIS will allow the safe haven to continue.

Does this mean that it is now necessary to step up the level of intervention, perhaps with Special Forces assisting the Iraqi Army to retake the territory that it lost six months ago? The Iraqi army will clearly have to work with Kurdish forces if they are to be effective, but I suspect that this is an essential ingredient in rebinding the Iraqi state in any event.

The failure of the Iraqi Army was bad enough, but the takeover of Syrian territory by ISIS poses a whole new order of difficulties.

To boil it down: If ISIS is defeated in Syria, who takes over?

The will be little western appetite in giving President Assad such a prize. Is there still a viable moderate Syrian opposition that could do so? They have also been routed by ISIS. So no easy solution there.

But this much is clear: So long as ISIS remain in control of large parts of Iraq and Syria, the risk of terrorism in western nations will be much greater than it would otherwise be.

After Paris is this a risk worth bearing? Do the 40 leaders who accompanied the million-plus French citizens in Paris have a responsibility to do more than march?

In my view they have the urgent need to ask themselves if they are prepared to allow ISIS to continue as a proto-state; a state which seems to have terrorism as core philosophical element of its national ethos.

Failing to grapple with ISIS will mean more incidents of terrorism. And that would be a failure of leadership.