Shamima Begum was 15 when she became radicalized, left her home in London for Syria and joined the Islamic State, marrying one of its fighters.

As the group’s grip on its last pieces of territory slipped, Ms. Begum, 19 and pregnant, fled to a refugee camp in northern Syria. When she met a British reporter there, she made one thing clear: She wanted to come home.

But Britain’s Home Office informed her family by letter of its plans to strip her of her citizenship.

The government says it is acting to protect the British public first. But a lawyer for Ms. Begum, who recently gave birth to a baby boy, said the move would render the British-born woman stateless.