“I lost everything but I did not lose my humanity,” he said.
Along with Mr. Aldeen and another Dane, Mohammed el-Abassi, who also worked for Team Humanity, three Spanish firefighters who volunteered for the Spanish group Proem-Aid faced as many as 15 years in prison.
The five were arrested on Jan. 14, 2016, just a few hours after successfully rescuing 51 migrants, according to Mr. Aldeen, the owner of the boat on which the five were working.
Not long after their operation, the men said, they had alerted the Greek authorities to another migrant boat in trouble, without approaching it. They were arrested soon after. “We didn’t even see the boat,” Mr. Aldeen had contended.
The prosecution drew condemnation from some Spanish officials and aid and advocacy groups, including Amnesty International, as Team Humanity and Proem-Aid sought to raise public awareness about the case.
“Since when is it a crime to save lives?” Team Humanity’s website asked.
Last month, the three Spanish firefighters — Manuel Blanco, José Enrique Rodríguez and Julio Latorre — held a news conference alongside Spain’s foreign minister, Alfonso Dastis, and the volunteers received backing from politicians, particularly in Andalusia, their home region.
Before the verdict, Cristina Morata, an official from the city hall of Seville, the capital of Andalusia, said she hoped that “common sense” would prevail after the men had already waited two years for a ruling.
A delegation of politicians from Andalusia traveled to Lesbos to show support for the defendants. Verónica Pérez, a Socialist politician who joined the delegation, said, “Humanitarian aid should never be condemned or sentenced but instead the opposite: It should be valued.”
In Denmark, Team Humanity had raised money to help cover Mr. Aldeen’s legal fees, accusing the Greek government of treating him like a criminal for trying to help refugees.
Mr. Aldeen has been met with great sympathy in Denmark. Yet other well-meaning Danes have similarly faced legal jeopardy. After the 2015 refugee crisis, hundreds of Danes were convicted of human trafficking for offering asylum seekers a meal or a ride from towns near the German border to train stations and ports with connections to Sweden.
Mr. Aldeen, the son of an Iraqi father and a Moldovan mother, is himself a former asylum seeker who left Moldova at the age of 9 and grew up in Denmark. He said he traveled to Greece in September 2015 after televised images of a drowned Syrian child, Alan Kurdi, who washed up on a Turkish beach, inspired him to help.
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The Kurdi family, like thousands of others who crossed or tried to cross the Aegean, were refugees from the war in Syria. Many, including children like Alan, continue to die.
“It changed my life,” Mr. Aldeen said, noting that he gave up plans to set up a small construction company and instead founded Team Humanity.
His lawyers called the case politically motivated. “I believe the defense we’ve put together is rather strong,” Mr. Aldeen’s Danish lawyer, Christian Dahlager, told Politiken, a Danish daily.
“But it’s obvious that from the side of the Greek authorities there’s a colossal interest in establishing that it is human trafficking or attempt of trafficking, as it deters others from doing the same,” he said.
The case joined an increasing number in Europe that have placed migrant rescue groups in the legal cross hairs. Italian authorities have searched and sometimes seized rescue boats over the past year, accusing them of abetting human smuggling and illegal immigration.
Earlier this year, Italian authorities seized the boat of a Spanish group, Proactiva Open Arms, that had picked up migrants in international waters and took them to Italy, instead of letting Libya’s coast guard take them back to North Africa.
In recent months, the often rapid intervention of the Libyan Coast Guard when migrant boats ran adrift has complicated the scenario at sea. Aid workers have reported threats, including gunfire that has been aimed at their vessels.
At the peak of the refugee crisis in late 2015 and early 2016, thousands of asylum seekers were arriving on the islands of the Aegean from Turkey every day.
Lesbos bore the brunt of that influx, prompting volunteers from all over the world to visit the island in a bid to help Greece’s strained coast guard officers. Greek authorities occasionally expressed concern, however, about the difficulty of coordinating with voluntary workers.
Arrivals from Turkey have increased in recent weeks with the improved spring weather, though the influx is far smaller than that at the peak of the crisis.
Greek authorities are now warning of a new potential crisis at the country’s land border with Turkey, where arrivals have increased threefold compared with last year, according to Greece’s migration minister, Dimitris Vitsas.