The UK Supreme Court ruled Friday that Shamima Begum, who left London as a teen to join the Islamic State in Syria, cannot return to the United Kingdom to fight the British government’s decision to revoke her citizenship. 

The ruling overturns a lower court judgment in July that said Begum, now 21, should be allowed to come back to her home country to participate in her citizenship case. But in November, the Home Office appealed the ruling on national security grounds, saying Begum’s return would “expose the public to an increased risk of terrorism.” 

In 2015, Begum and two classmates from east London’s Bethnal Green neighborhood boarded a flight to Istanbul and from there slipped across the border into Syria. Shortly after arriving in the Islamic State capital of Raqqa, then-15-year-old Begum married a Dutch militant. They had three children, all of whom died as infants, she said.

Begum, who is currently detained in a Kurdish-run displacement camp in Syria, spurred an intense debate over "Islamic State brides" who traveled to Syria and Iraq to join the terrorist group and whether Western governments should repatriate and prosecute them.