Even in the four European Union member countries that initially opposed a modest 120,000-migrant per nation resettlement quota — Poland, Slovakia, the Czech Republic and Hungary — voices were raised in support of the migrants, who were to be distributed across the 28-member bloc. Though a quota system has yet to take full effect, a recent appeal, signed by former presidents and prime ministers and other prominent Europeans, many from those four countries, called on their countries to abandon their hostility to the migrants and remember their own recent past.

That might be a hard sell, however, in a Europe already preoccupied with terrorist recruitment among disaffected Muslim populations from earlier, much smaller migrations.

“Throughout Europe, xenophobia and open racism are running rampant, and nationalist, even far-right parties are gaining ground,” Joschka Fischer, the former German foreign minister, wrote recently in an article that appeared on Project Syndicate, an online news service.

“At the same time, this is only the beginning of the crisis, because the conditions inciting people to flee their homelands will only worsen. And the E.U., many of whose members have the world’s largest and best-equipped welfare systems, appears to be overwhelmed by it — politically, morally and administratively.”

Those stresses pose a challenge for the future, experts say, because the flow is unlikely to ebb anytime soon.

“I don’t think this wave can stop,” said Sonja Licht of the International Center for Democratic Transition. “It can maybe from time to time be somewhat less intensive, we simply have to prepare. The global north must be prepared that the global south is on the move, the entire global south. This is not just a problem for Europe but for the whole world.”