RITSONA, Greece—With walled camps and tougher border controls, Greece is hardening its approach ahead of summer when migrant arrivals pick up, defying criticism from aid groups and saying it has little choice given a lack of support from the rest of Europe. The squalid conditions facing many asylum-seekers were laid bare last year when a fire devastated the sprawling Moria camp on Lesbos, and Greece has denied repeated accusations that its coast guard vessels have pushed back migrant boats as they entered Greek waters from Turkey. Migration Minister Notis Mitarachi says the government is taking a tougher approach “so we don’t send the wrong message of incentivizing people to come” to Greece. “Our policy is strict but fair,” Mitarachi told Reuters. Greece was the frontline of Europe’s migration crisis in 2015, when a million refugees from Syria, Iraq, and Afghanistan landed. The numbers have slowed sharply since, but Greece says …