When I was a counter-terrorism operative in MI6 -- Great Britain’s equivalent to the CIA -- the work was exhausting, intense, highly dangerous, under-resourced, and often extremely debilitating.

Successes, and there were so many, rarely lifted the spirits of spies like me because there was always the pervasive Sword of Damocles hanging over operatives that failure at some point was inevitable. 9/11, Madrid, 7/7, Tunisia, Sinai, and now the awful events in Paris, are referred to as “the ones that slipped through the net”.

I find myself using that phrase in interview, elaborating that for every one failure, thousands of planned terror attacks are thwarted. But, though that ratio is accurate, it is undeniably a statement that intelligence agencies cannot protect us from everything at all times.

Intelligence operatives know that they will miss something. It’s inevitable.

The terror threat is evolving on an arc of near unstoppable destruction and evolution.

My thoughts and condolences are with the family and friends of the 130 dead in Paris and over 200 injured. My thoughts are also with French intelligence officers and their professional allies.

France has world-class counter-terror capabilities. Their officers will be viewing the aftermath of last Friday’s attacks with a feeling of utter sorrow and unfair self-blame.

The terror threat is evolving on an arc of near unstoppable destruction and evolution.

Organized amateur terror, easy to spot and destroy, has been quashed.

Small cell units, personified in Al Qaeda, were a tougher challenge but have for the most part been dismantled.

Then came lone wolf terrorists, and they remain a threat; radicalized and directed remotely by preachers and hate mongers who arouse warped purpose in predominantly youthful and testosterone laden males.

Islamic State (ISIL) is the latest chapter and a step up in the evolving terrorist arc, operating on quasi-military principles, orthodox and unorthodox.

If Syria and northern Iraq is Islamic State’s conventional battleground, Europe is the series of borders it crosses to operate in unconventional roles behind enemy lines.

The perpetrators of the attacks in Paris were skirmishers, different backgrounds and locations, though all Muslim in heritage; recruited and brought together for shock attacks that caught the intelligence world on guard but frazzled with exhaustion.

America is ISIL’s prize target, for sure. And it will soon be attacked.

When Americans are killed in due course, it won’t be the fault of the CIA, NSA, FBI, and other agencies. They will have done their utmost and more to stop attacks.

And it is not just a case of throwing extra resourcing and cash at the problem. No doubt that helps, but it takes time to train up spies to become operationally deployable.  And when they are fully skilled, what do we want them to do on our domestic territories?

America and European countries aren’t totalitarian states. We don’t want all pervasive Big Brother surveillance and other forms of intrusion to try to identify the terrorist amongst us. The outcry from Edward Snowdon’s revelations about the NSA’s PRISM program of email intercept showed that.

In hostile countries, and very alone, I put my life at risk to protect my country’s values – democracy, freedom of speech and movement, respect, civility, art, and Enlightenment sensibilities, amongst others.  I wouldn’t have done what I did on behalf of the USSR, Iran, North Korea, and Syria.

So, to send our brave intelligence officers and military to the shifting sands battlefront of terrorism requires us to know what we’re fighting for.

ISIL is fighting for cruelty, a wannabe Middle Eastern caliphate where women and children are raped and butchered by its so-called Muslim protectors, and it is cleverly taunting us away from our values. It wants us to live in police states, our values crumpled.

World War II, far more so than subsequent wars, is our template. Against tyrannical aggression, what matters to us? What do we stand for? What are we prepared to do that doesn’t make us as bad as the enemy?

ISIL won’t ultimately win the war, but it will win far too many battles. Overseas, they are operating like the South African commandos of the Boer War – disparate fellows, not military in appearance, rapid and clever in action, but not necessarily clever in mind and soul.  Intelligence agencies may spot some of these men in advance of their plans, but they can’t get to all of them unless we allow a security blanket to be placed over us.

To defeat ISIL, aggressive military action in Syria is a key action that doesn’t denigrate our principles.

It is complicated, not least because of Russia’s support for Syria’s President Assad. It is to be hoped that after the G20 summit, whose topics were dominated by the Paris slaughter, will find a roadmap forward, though I’m not hopeful that action will translate beyond the rhetoric of state leaders.

So too, we must bravely reconcile our crucial desire to be multicultural with a liberalism that allows people into our countries who hate us.

I’m a single parent of two children, and maybe that’s more important in this context than my covert previous life.

My son, age 11, will never grow up to be a terrorist because I’ve devoted time to give him a moral compass. Many of the young men in ISIL’s ranks have no idea about the inter-state complexities and Sunni-Shia antipathies in the Middle East. They just want a fight. Wherein their parents’ good upbringing?

The only way to ultimately eradicate Muslim anarchists is to stand by our liberal values and be schoolmasterly to those who don’t have the backbone to raise their children in a way that is respectful of borders, law, and peace.

In the meantime, let's take the fight to them and wipe out the immediate problem.

Matthew Dunn was a former MI6 officer who worked in hostile locations around the world. He is the author of the espionage novels "Slingshot," "Spycatcher," "Sentinel," "Counterspy," "Dark Spies," "Spy House," and the forthcoming "A Soldier’s Revenge" (William Morrow). For more information visit matthewdunnbooks. You can also follow Mr. Dunn on Twitter @MatthewHDunn and find him on on Facebook