A jihadist group that has already seized control of large swaths of Syria and Iraq extended its territorial claims into Lebanon on Sunday, taking over a small city there after overrunning police posts and a military barracks.

Fighters from the extremist group, which calls itself the Islamic State, streamed across the border from Syria in what Lebanon’s defense minister called a premeditated attack after one of its commanders was arrested by Lebanese security forces on Saturday near the city of Arsal.

By Sunday night, Lebanese officials said the militants had overtaken Arsal, a city of 35,000 Lebanese which also houses 100,000 Syrian refugees, but didn’t free the commander. […]