The United States has begun delivering nearly $20 million of arms including assault rifles, anti-tank missiles and mortars to bolster Lebanon’s army after Islamist insurgents seized a border town for several days this month, U.S. officials said on Friday.

Lebanon, a country of about 4 million which borders Israel, has long been buffeted by the rivalries of regional and international powers including Iran and Saudi Arabia, who back its opposing politicians largely along sectarian lines.

But the threat of expansion by militants who want to build a “caliphate” stretching fromIraq to the Mediterranean has created an unusual common front among Lebanon’s otherwise normally fractious political forces against radical Islamists. […]