Defense Secretary Jim Mattis declined to discuss timeframes for the mission in Afghanistan after meeting with defense ministers in Brussels on Thursday over the future of America's longest war.

"I don't put timelines on wars, it's that simple," Mattis told reporters. "War is a fundamentally unpredictabe phenomenon, and every effort to create a pat answer to something like that is probably going to fail."

Mattis is using his time in Brussels to discuss NATO commitments for the fight in Afghanistan as he prepares a plan for the next phase of the war. He'll also use information gathered from Joint Chiefs Chairman Gen. Joe Dunford, who visited Afghanistan this week.

On Thursday, Mattis was asked to respond to Director of National Intelligence Dan Coats' testimony before lawmakers last month, in which he said the situation in Afghanistan "will almost certainly deteriorate" through 2018.

Coats told members of the Senate Intelligence Committee that the deterioration will continue "even with a modest increase in military assistance by the United States and its partners."

Mattis said of the comments "They're entitled to their assessment," adding that it's more important to ask what would happen if the U.S. pulled its troops from the country.

"You can't say, 'well I got tired of it so I'm going to come home' and wonder why you get hit again. We've all seen the results of leaving ungoverned areas. What is the price of not fighting this war, and in this case we're not willing to pay that price."