The Western-backed effort to drive the Islamic State out of Iraq is making headway. The self-proclaimed caliphate has lost two-thirds of its territory. The battle for Mosul, Iraq’s second city, is well underway.

But there is a dark side to the coalition’s success in Iraq. We’ve seen it in the streets of Paris, Nice and London: The more the Islamic State is cornered, the more of its sympathizers commit terrorist attacks in the West.

Gilles de Kerchove, the EU’s counterterrorism coordinator, has warned that as Islamic State leaders are killed and the group loses territory in the Middle East, it could take the fight to Western Europe.

Returning jihadists, who are estimated to number in the thousands, pose a particular threat. Not all plan to commit attacks upon returning, but the risk that they do is substantial and more fighters could return in the coming months as the Islamic State is reduced.

What can governments do?

The first thing European governments must do is allow their law enforcement and spy agencies to share criminal records and intelligence. Europe’s open borders have to be paired with an open exchange of information.

Europol has taken the lead, establishing a classified database to monitor foreign fighters who have returned to Europe. It allows national authorities to crosscheck information from multiple states and tracks criminal activity which, in turn, could provide clues as to who is plotting an attack.

Another priority is border security. The EU has increased funding for Frontex, enabling it to hire more personnel and assist national border guards in conducting security checks.

Frontex has also taken the lead in using new technology to detect falsified passports.

There is no magic bullet to prevent terror attacks, especially when they are carried out by so-called lone wolves. But there are things governments can do — and they must be done before the Islamic State becomes desperate.