Six months after the death of Anna Campbell, who in 2018 became the first British woman to be killed fighting alongside Kurdish forces in Syria, her dad is pottering around his garden in Lewes, East Sussex, looking lost. “I feel a lot of guilt,” Dirk says. “A lot of grief. I feel her loss all the time.” At a family lunch, the big, loving, garrulous kind so many dream of, he comforts his daughter Hester as she weeps at the table. “I’m not in a good way,” she admits. Then, in an interview to camera: “I feel robbed … like this could have been intercepted.”

Could it, though? In never-before-seen footage, we see Anna at the international training academy of the YPJ, the all-female Kurdish militia she joined in northern Syria in 2017. Dressed in military fatigues, kneeling on a Persian rug, she looks fresh-faced, happy and exactly where she wants to be. “I never thought I could be someone who could participate in the revolution here,” she says. “I don’t regret it for a second.” Less than a year later, she was killed by Turkish airstrikes in the besieged city of Afrin.

In Anna: The Woman Who Went To Fight Isis (BBC Two), which was as restrained and respectful as you would hope a documentary on such a complex subject would be, Dirk tries to make sense of his daughter’s death by reliving her life.

He travels, as Anna did, from Iraq across the Tigris river into wartorn Syria, then north to Rojava, where she joined the YPJ, a western ally in the war against Isis. He meets some of the volunteers who continue to come from all over the world, for all sorts of reasons, to fight alongside Kurdish guerrillas. He sees where his daughter slept beside her Kalashnikov. He eats with her fellow revolutionaries. He reads her notebooks in which she recalls “the endless cans of Pepsi and AK-47s” and how badly she wants to fight. Whether any of it helps becomes a moot point. “The more I know, the more I feel close to her,” explains this gentle, thoughtful, magnanimous man who, like many grief-stricken people, is too hard on himself.

In a lesser documentary, Anna may have remained a naive figure who died a futile death fighting someone else’s war, which is often how she was portrayed by the media. Instead, a picture emerges of an emotional and committed activist, the type of child who defended a bumble bee being tormented in the playground by her classmates. “That was her soul’s calling … to protect and nurture,” Dirk says. She inherited her activism from her mother, Adrienne, who died in 2012 from breast cancer. This is a family felled by grief.

Two days after her mother’s death, Anna went to an anarchist book fair where there was a meeting about Rojava, then at the centre of the Kurdish fight for independence. Its objective – to establish a free feminist society – deeply affected her. By that time, Anna had dropped out of university and was a full-time activist engaged in direct action in anti-fascist and hunt saboteur movements.

In May 2017, she announced that she was going to Rojava. “I didn’t question her,” Dirk recalls. “All I could say was … it’s been nice knowing you.” He reflects on his own childhood, growing up with a mother who thought “I would never amount to anything”. How, after a school performance, she said to him: “I was so embarrassed I felt sick.” He swore he would do things differently with his own children. He would respect them, embrace them, let them make their own decisions.

Back in Syria, Dirk meets Nesrin Abdullah, the YPJ senior commander who gave the order for Anna to go to Afrin. “I was deeply affected by how honest, pure and clear she was,” Abdullah says. She didn’t want to send Anna to fight, but Anna started phoning every day. She dyed her hair black to look Kurdish.

In a video recorded before leaving, she says: “I’m very happy and proud to be going to Afrin.” “It was her choice,” Dirk concludes. “She wanted to be the person she became.”

Back home, the morass of grief and guilt goes on. What is just as engulfing is the sense of Anna as an exceptional person and the equally exceptional family who continue to mourn her.

“How could you willingly cause everyone you know so much pain?” her sister Rose wonders. “But it’s bigger than that. She’s braver than that. The world has so much going on in it and it’s bigger than our tiny pieces of grief.”