Shamima Begum, who left London in 2015 to join ISIS, cannot return to Britain while she fights to restore her citizenship, the U.K. Supreme Court ruled on Friday. Begum was 15 when she ran away to Syria with two friends; she's now being held in a detention camp in northern Syria.

Begum was born in the U.K., but the country revoked her British citizenship two years ago, citing security concerns. She then asked for permission to enter the U.K. to appeal that move, but the government denied her application.

Soon after joining the Islamic State, Begum married a Dutch fighter in the terrorist group. Now 21 years old, she has given birth to three children, all of whom have died due to illness or poor conditions. Her husband, Yago Riedijk, is in a Kurdish-run detention center elsewhere in northern Syria, according to the BBC.

Begum's flight from the U.K. to join the terrorist group triggered an international search. Four years later, a journalist found her in a detention camp. At the time, she said she did not regret her actions. But she also said, "I actually do support some British values and I am willing to go back to the U.K. and settle back again and rehabilitate and that stuff."

When Begum's citizenship was revoked, former Home Secretary Sajid Javid said she might be able to pursue citizenship in Bangladesh, citing her eligibility because of her mother's origins. As NPR's Matthew Schwartz reported, "The British Nationality Act of 1981 lets the government strip Britons of their citizenship if it would be 'conducive to the public good' and if the person wouldn't become stateless as a result."

But Bangladesh has shown no interest in extending Begum citizenship, noting that she's never been there or sought a Bangladeshi passport. Begum has agreed with that assessment.

Begum's legal dispute with the U.K. has been convoluted, as she followed several avenues of appeal. At one point, the U.K.'s Special Immigration Appeals Commission found that the government had not violated its (own) policies when it stripped Begum of her nationality.

But while the commission noted that Begum's appeal would not necessarily succeed, it also said that because of her status living under armed guard in the Al-Roj camp run by the Syrian Democratic Forces, her appeal process could not be seen as fair or effective.

The U.K. Court of Appeal later sided with Begum, saying she must be allowed to enter the U.K. to fight to restore her citizenship. But the government appealed to the Supreme Court, which today ruled against Begum's return.

The Supreme Court eviscerated the Court of Appeal's ruling, laying out four points in which it said the lower court was outright mistaken or applied faulty reasoning.

For instance, the Supreme Court stated in a summary of its judgment, "the Court of Appeal mistakenly believed that, when an individual's right to have a fair hearing of an appeal came into conflict with the requirements of national security, her right to a fair hearing must prevail. But the right to a fair hearing does not trump all other considerations, such as the safety of the public."

The Supreme Court also said the lower court had erred by invoking a British law guaranteeing residents access to the courts, despite Begum not fully making that argument. And it said the court had not given the government's national security assessment "the respect which it should have received."

If it's impossible for Begum's case to be fairly heard, the Supreme Court said, her appeal should have been stayed until she could effectively participate in hearings.

"That is not a perfect solution, as it is not known how long it may be before that is possible," the court stated. "But there is no perfect solution to a dilemma of the present kind."

Copyright 2021 NPR. To see more, visit https://www.npr.org.