BARCELONA, Spain — A year ago, Oscar Camps, a Spaniard, was a lifeguard who traveled with a colleague to the Greek island of Lesbos to see what he could do to help as thousands of Syrian refugees washed up from Turkey.

He had tried to volunteer for different European organizations and embassies, but was turned down, and arrived on the island with little more than swim fins.

Mr. Camps said he had no idea how critical the situation in Lesbos had become.

Within two hours of their arrival, he and his friend were taking off their shirts and shoes to jump into the sea to save people from drowning, after watching a boat sink close to the island’s rocky northern coast.

Lesbos “changed my view on almost everything,” Mr. Camps recalled in a recent interview.

“I realized that Europe’s inaction was absolutely deliberate — a decision by the European Union to use the Mediterranean and the Aegean as dissuasive tools, to send the message to people that they shouldn’t try to come because they would drown.”