He led a band of armed rebels. She was an expert in forging documents. They robbed banks, they staged prison breaks and they were in love.

It was the early 1970s, and José Mujica and Lucía Topolansky were members of a violent leftist guerrilla group, the Tupamaros. To them, their crimes were justified: They were fighting a repressive government that had taken over their small South American nation, Uruguay.

He was 37 and she was 27 when, during a clandestine operation, they first came together. “It was like a flash of lighting in the night,” Mr. Mujica, now 89, recalled many years later of their first night together, hiding out on a mountainside.

Amid war, they found love. But just weeks later, they were thrown in prison, where they were subjected to torture and abuse. Over 13 years, they managed to exchange only a single letter. The guards confiscated the rest.

In 1985, Uruguay’s dictatorship ended. They were released on the same day, and they quickly found one another.