Last year, Mr. Santrich was among the most prominent FARC members to return to battle.

“It was a necessity,” Ms. Marín said of the 2016 peace deal. “Because the war lasted 50 years, and we were just killing the same people as always. And because we realized that with bullets we were not going to solve anything.”

But now she is not sure how long the deal will last — or how she will endure as a civilian.

Ms. Marín was born in the city of Tuluá, in western Colombia, outside of Cali. Her mother, a homemaker, gave birth to seven children. Ms. Marín was the youngest and the only girl. Her father owned a fruit market.

They were not rich, she said, but they were not desperately poor either.

As a child she wanted to be a police officer, attracted to power. “It was that idea of authority,” she said. At age 8, the government raided her home, she said, having discovered that her older brother was involved with the FARC.

The family fled, first to a farm and later to a small town, and spent the next few years evading the police, the military and paramilitary groups. She began to think of the FARC as “the good ones,” following a childhood in which she’d always viewed them as “the bad ones.”

By age 12 her relationship with her father had collapsed. He was domineering and abusive, and she was defiant and outspoken. “In my house you would do what my father said, because he was the authority,” she said. For three years Ms. Marín did not speak to him.

At age 15, nearing high school graduation, she saw three possible futures for herself: life with a man like her father, life with drugs or life with the guerrillas.

She chose the rebels. Looking back, she said, it was without a doubt a feminist flight.