US Secretary of Defence James Mattis gave possibly the quote of the year in a recent interview when asked, "what keeps you awake at night?"

Speaking to John Dickerson on CBS‘ Face The Nation, ‘Mad Dog’ Mattis said “nothing” keeps him awake at night and that he’s the one who keeps other people up.

One thing not keeping Defence Secretary awake at night is the impending threat of war with North Korea - though he did say if it were to happen it would be "catastrophic".

"A conflict in North Korea, John, would be probably the worst kind of fighting in most people's lifetimes," he told host John Dickerson.

"Why do I say this? The North Korean regime has hundreds of artillery cannons and rocket launchers within range of one of the most densely-populated cities on Earth, which is the capital of South Korea.

"We are working with the international community to deal with this issue, this regime is a threat to the region, to Japan, to South Korea, and in the event of war they would bring danger to China and to Russia as well."

On NATO

Mattis reaffirmed the US' commitment to NATO, despite Trump calling it "obsolete" during the presidential campaign.

He said that the question is not whether the United States is with NATO, but instead how the US and NATO can work together.

Issues they can work on include the "various forms" of attack that NATO allies face, Mattis said. They range from an active insurgency in Turkey to attacks like the one last week in Manchester.

"We have seen the terrorist threat, especially against the southern nations of NATO," Mattis said.

On ISIS

Discussing the war on terror, Mattis said the US policy towards ISIS was now "annihilation".

“We’re not the perfect guys, but we are the good guys. And so we’re doing what we can," he said, adding that civilian casualties are "a fact of life in this sort of situation”.

“Our strategy right now is to accelerate the campaign against Isis. It is a threat to all civilized nations. And the bottom line is we are going to move in an accelerated and reinforced manner, throw them on their back foot.

“We have already shifted from attrition tactics, where we shove them from one position to another in Iraq and Syria, to annihilation tactics where we surround them. Our intention is that the foreign fighters do not survive the fight to return home to North Africa, to Europe, to America, to Asia, to Africa.

"We’re not going to allow them to do so. We’re going to stop them there and take apart the caliphate.”

He said that the fight against ISIS would be a long one, and would ultimately be "a fight about ideas".

“We’re going to shatter their sense of invincibility there in the physical caliphate [...] We have to dry up their recruiting."