Islamic State militants are threatening to overrun a key province in western Iraq in what would be a major victory for the jihadists and an embarrassing setback for the U.S.-led coalition targeting the group.

A win for the Islamic State in Anbar province would give the militants control of one of the country’s most important dams and several large army installations, potentially adding to their abundant stockpile of weapons. It would also allow them to establish a supply line from Syria almost to Baghdad and give them a valuable position from which to launch attacks on the Iraqi capital.

The Islamic State’s offensive in Anbar has received less attention than its assault on the Syrian border city of Kobane, which has played out in view of news photographers standing on hills in nearby Turkey. But in recent weeks, Islamic State fighters have systematically invaded towns and villages in Anbar, besieged army posts and police stations, and mounted attacks on Iraqi troops in Ramadi, the provincial capital. […]