RAS BAALBEK, Lebanon—One of the lesser-known chapters of Islamic State’s occupation of parts of the Middle East drew to a close on Tuesday, with Lebanon reclaiming all the territory that the extremist group controlled in the country for the past three years.

Lebanese forces said they had captured the last sliver of land held by Islamic State after they launched a ground operation this month to clear the militants from the roughly 120 square kilometers of rugged mountain terrain near the border with Syria where they were based.

“All the lands are now Lebanese and secure,” Lebanese Army Brig. Gen. Fadi Daoud, who commanded troops during the ISIS offensive, said in an interview on Tuesday.

Lebanon is the latest in a string of military defeats for Islamic State. The extremist group has lost more than half of the territory it captured three years ago in a blitz across Iraq and Syria.

It recently lost its Iraqi stronghold of Mosul and was defeated this past weekend in the strategic town of Tal Afar near Iraq’s border with Syria. The militants are also rapidly losing ground in Raqqa, their de facto capital in Syria.