(CNN) Terror attacks will happen; some just will. No matter how hard authorities fight to intercept plots, they can't stop them all -- and ISIS has made their jobs even tougher, says Sir John Sawers.

In James Bond movies, the head of the MI6 is called "M." In reality, that person is code-named "C" for Chief of Secret Intelligence Service. Sawers was "C" until he stepped down last year.

He did his best to keep Britain terror-free, and mostly, he was successful, he said on CNN's Global Public Square . But the dangers have grown since he left his post. "I think it's pretty chaotic and dangerous at the moment," Sawers said.

Olympian task

On Sawers' watch, London hosted the Olympic Games in 2012, and security was already a huge job then, but it was manageable. That may no longer be the case.

"We were pretty confident that the London Olympics would be terrorism-free. And thanks to a lot of hard work, it was," Sawers said. "I don't think you could be quite so confident now if the London Olympics were in 2016, for example."

Since ISIS has come along, terrorists have changed their game. "They're not trying to fly airliners into buildings. They're doing simpler things," he said.

They're picking up Kalashnikovs, pistols or knives and walking into places like the offices of French satire magazine Charlie Hebdo, where two gunman killed 12 people in January.

Or into a market, or onto a crowded passenger train, like the man tackled last month on the Amsterdam-to-Paris line as he brandished a rifle, a pistol and a box cutter.

"That's much harder to stop and obstruct as an intelligence service," Sawers said.

ISIS recruiting is a worry, not migrants

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Though many terrorists in the West are disaffected younger people, often from immigrant families, Sawers doesn't see significant dangers from the current mass migration from Syria, Iraq and other war-torn countries.

"I think the great bulk of these refugees are people genuinely fleeing conflict, fleeing for their lives and seeking a better life for themselves and their families," he said.

What does worry him is the fact that European citizens are signing up with ISIS.

"They can come back radicalized and keen to carry out terrorist attacks in their home countries," he said.

To fight the new dangers, intelligence services need to gain the trust of Muslim leaders in their home countries and sneak secret agents into overseas terror organizations, Sawers said.

In spite of the dangers, he said, he believes Western intelligence has done a good job at thwarting attacks so far. It's just that 100% success is unrealistic.

Obama 'cautious, possibly to a fault'

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Sawers would have liked to have seen U.S. President Barack Obama more involved militarily in the fight against overseas terrorism, but he thinks Obama's focus on weighty domestic issues and sentiments may have held him back.

"I think on his external policy -- I think he's been cautious, possibly to a fault," Sawers said of Obama. "I think he's been hesitant to get back involved in military engagements in the Islamic world, for example, scarred by what had happened in Iraq and Afghanistan and recognizing that public opinion and the political appetite for further engagements is very low."

But he praised Obama for reducing al Qaeda's abilities to launch attacks. The operation that took out Osama bin Laden was daring, he said.