Ahn Hak-sop leaned on a cane and a dining table as he lowered himself at glacial speed to sit on the floor of his home. The husk of a man who once loved judo, he has been worn down by life in South Korea, an enemy nation that locked him up for more than four decades.

His speech was slow and slurred because of his dentures, but Mr. Ahn was eager to explain why he so hated the United States. From the words he used — “comrades,” “struggle,” “imperialism,” “colony” and “independence”— there was no mistaking the former North Korean soldier’s devotion to communism.

“I am still trying to figure out this thing called capitalism,” said Mr. Ahn, 95. Along the walls around him were papier-mâché figures mocking Uncle Sam and the Statue of Liberty as money-loving, machine gun-toting, bloodthirsty warmongers. “People in South Korea don’t realize that they are slaves in a colony and their leaders can’t do anything without American approval.”

The Korean War ground to a cease-fire 72 years ago, after millions were killed. But it never ended for Mr. Ahn, who as a young man fought on the North Korean side. He was captured by the South during the conflict and then survived 42 years and four months in prison on espionage charges, mostly in solitary confinement. Released in the mid ’90s, he stayed on in the South to continue to campaign for his life’s mission: the removal of U.S. military from the Korean Peninsula.

These days, Mr. Ahn is waging the last battle of his life: He wants to return to North Korea to die in his political and ideological home.