“What I want is to make sure that people can decide their own fate,” Mr. Glezos said.

It was a point he planned to press in Brussels, along with what he called an “anti-government, anti-system and anti-troika” message, referring to the three international lenders that gave Greece two multibillion-dollar bailouts in exchange for tough austerity measures. Mr. Glezos claims those terms benefited banks while crushing living conditions for average Greeks.

Occasionally, Mr. Glezos can sound extreme, even within the Syriza party, which once threatened to take Greece out of the eurozone. He rails about Germany’s “colonization” of Europe and has demanded that Berlin pay Greece war reparations of up to a trillion euros, much more than the entire cost of Greece’s bailout.

Yet if Mr. Glezos is strident, it is because he believes that “forces of oppression” should not prevail, a conviction that gripped him the moment he and Mr. Santas sneaked into the Acropolis.

That night the two men, then 18, crept into a cave beneath the Acropolis, armed with only a lantern and a knife, and silently made their way to the flagpole while unsuspecting German officers drank toasts near the Parthenon to celebrate Hitler’s takeover of Crete. After bringing down the Nazi flag, they cut it into pieces and buried it in a hole. When Mr. Glezos returned home, his mother grabbed him and demanded to know where he had been.

“I opened up my shirt and pulled out a piece of the swastika,” Mr. Glezos recalled. “I showed it to her and said, ‘That’s where I was.’ Without saying a word, she hugged me and left.”

The next day, Mr. Glezos’ stepfather asked his mother what her son had been doing. “Look at the Acropolis, and you will know,” his mother replied. A few hours later, the Nazis announced the death penalty for the perpetrators.

“That was my first act of resistance, and I knew there would be others,” recalled Mr. Glezos, who was captured by the Germans in 1942 and thrown in prison.