"Migration is now at the top of the political agenda, which it didn't use to be before 2015," said Camille Le Coz, director of the Migration Policy Institute Europe, noting changing EU alliances. "We are seeing a shift toward the right of the political spectrum."

Greece was reeling from a crippling economic crisis. The influx added to anger against established political parties, fueling the rise of once-fringe populist forces.

EU nations fought over sharing responsibility for asylum-seekers. The bloc's unity cracked as some member states flatly refused to take migrants. Anti-migration voices calling for closed borders became louder.