Existing EU policies to control who is coming to Europe appear in tatters. Ms. Merkel and the European Commission, the EU’s executive arm, have staked much on the idea of registering migrants at processing centers, dubbed “hot spots,” in Greece and Italy, and in distributing them around the EU under a quota system. But the distribution idea now looks even harder to implement in the face of political doubts from many countries. And the registration of hundreds of thousands of migrants’ personal details, even their fingerprints, appears in the wake of Paris to be an ineffectual way to identify or stop Islamic militants.
In Germany, the attacks quickly galvanized opponents of Ms. Merkel’s open-door refugee policy. Critics said that it had become clearer than ever that the migrants from the Middle East represented a grave security threat.

“It cannot be that we are not protecting our borders and that immigrants from Syria are arriving without being registered,” Alexander Gauland, a deputy chairman of the populist Alternative for Germany party, said in an interview. “Ms. Merkel’s policy of opening the border for everyone from this region simply cannot stand anymore.”

German police who have been deployed along the border in recent months already take fingerprints scans of migrants and cross-check them against international criminal databases. But critics say the process still leaves too many loopholes, and the sheer scale of migration increases the risks that terrorists will enter.