Ms. Campbell, whose mother died several years ago, grew up in East Sussex, in southeastern England. She studied at Sheffield University, but academics did not suit her, her father said, and she dropped out, worked as a blacksmith and trained as a plumber.
She traveled to Syria last year, received military and Kurdish-language training, and joined the Women’s Protection Units, or Y.P.J., the all-female military forces that fight alongside the Y.P.G. She died her blond hair black, to blend in better, and took a Kurdish name.
“I wanted to participate in the revolution of women that is being built up here and fight, and join also the weaponized fight against the forces of fascism and the enemies of the revolution,” she said in a video the Y.P.J. posted on Facebook on Monday. “And so now I’m very happy and proud to be going to Afrin to be able to do this.”
“She knew the risk she was facing,” said Ahed Elhendi, an adviser in Washington to the Syrian Democratic Council, the political group that controls northeast Syria and is allied with the Y.P.G. “When Afrin was being shelled by the Turkish Army, she asked to join the fight there many times until she finally made it there.”
Mr. Elhendi, who said he met Ms. Campbell last October in Syria, added, “We thought she could be a great asset to voice our dilemma, but her answer was always, she came for resistance, to fight.”
The Syrian war, now seven years old, features a complex array of forces. Kurdish-led militias, backed by the United States, are fighting another American ally, Turkey, which claims that the Y.P.G. is an extension of a Kurdish insurgent group, the Kurdistan Workers’ Party, also known as the P.K.K., that has been labeled a terrorist organization.
The Kurds are also fighting the Syrian government of President Bashar al-Assad, which is backed by Russia, Iran and Iranian-sponsored Shiite militias, and has increasingly reasserted control over the country.
Each of those factions has, to varying degrees, also fought the Islamic State group, now all but eliminated as a military force in Syria. And other rebel groups round out the picture, including Sunni Islamists who oppose all the major parties.
Mr. Campbell, 67, a musician, said he tried to talk his daughter out of going to Syria, knowing that she could be hurt or killed. “In retrospect, I could have made the argument more forcefully, but I think she would have dug her heels in even harder,” he said.
“I knew she was tough, but I never saw her as pointing a gun,” said Mr. Campbell, who learned of his daughter’s death on Sunday. “Now the bottom has dropped out of my world.”