FBI director James Comey is warning that even if ISIS is defeated in Syria, it won't make the West any safer.

Comey believes that hundreds of ISIS-trained terrorists would fan out across Europe and the U.S., continuing the fight even if the Islamic State ceases to exist as an organized force.

New York Times:

“At some point there is going to be a terrorist diaspora out of Syria like we’ve never seen before,” Mr. Comey said at a cybersecurity conference at Fordham University. “Not all of the Islamic State killers are going to die on the battlefield.”

Mr. Comey predicted that the military coalition would eventually succeed in crushing the Islamic State, but that “through the fingers of that crush are going to come hundreds of really dangerous people and they are going to flow primarily to Western Europe.” But some, he said, could well end up in the United States.

He drew a parallel between this scenario and the formation of Al Qaeda more than a quarter-century ago, which drew from fighters who had been radicalized fighting in Afghanistan in the 1980s and early 1990s.

“This is 10 times that or more,” Mr. Comey said. “This is an order of magnitude greater than anything we’ve seen before.”

Mr. Comey said the American public got a glimpse of what this threat would look like in the terrorist attacks in the past year in Brussels and Paris. “We in the American counterterrorism business are constantly focused on that — that’s not here yet, but that challenge is going to come,” he said.

ISIS fighters returning home in great numbers will almost certainly have terrorist infrastructure already in place in Europe's major cities.  Financing, intelligence, weapons procurement, transportation – all would be available to fighters who wished to carry on their jihad against the West.  This infrastructure would allow them to plan sophisticated and deadly attacks resulting in mass casualties, as we saw in Brussels and Paris.

Comey is right to worry, but it would be a great achievement to destroy the Islamic State's base of operations in Syria and Iraq.  As in the reduction of al-Qaeda in Afghanistan after 9/11, it won't stop the terrorists in their tracks.  But it will make it more difficult for them to attack us.

But before that happens, the wave of attacks in Europe will continue and probably wash up on our shores before long.