The mass displacement of so many young men poses great challenges to countries like Syria, home to more than half of those fleeing: The exodus deprives them of a demographic vital to reconstruction and economic growth.

There are also great risks for Europe, which has long struggled to assimilate immigrants and could face the creation of a new underclass that taxes the public purse. Many also worry that pockets of radicalization could grow if the aspirations of the new arrivals end in isolation and poverty.

“We know on the positive side that migration can boost economies and trade and lead to cultural exchange,” said Lado Gvilava, the head of the International Organization for Migration in Turkey, the departure point for most migrants. “But if it is mismanaged, it becomes a problem for both the receiving states and the countries left behind.”…

To succeed, Europe must invest heavily in programs to teach migrants languages, pair skilled workers with jobs and help everyone else find training and work, said Demetrios G. Papademetriou, the president of the Migration Policy Institute Europe.