Belgium’s relatively small security force is widely regarded as among the least effective in Europe, and it has struggled to penetrate the country’s homegrown Islamist network or track the movements of Islamic State veterans.

It took Belgian officials four months to locate Abdeslam, the Brussels native and alleged lone survivor of the cell that carried out the Paris attacks on Nov. 13. Europe’s most-wanted fugitive was finally captured March 18, even though he had been hiding out in the Belgian capital, not far from his childhood home…

Because of Europe’s open borders, Belgium’s terrorism problem now threatens all its neighbors, Hoffman said.

“In Europe, individual countries are only as strong as their most vulnerable neighbor,” Hoffman said. “You can have a strong surveillance network and counterterrorism machinery, but if a neighboring country doesn’t have the same resources or the same perception of the threat, that becomes a launchpad that can be exploited by groups that want to project terrorist attacks elsewhere.”