In late April, a commander for Islamic State said his forces were ready to launch an offensive to take Ramadi, and the group called for fighters to redeploy to Iraq from Syria.

Three weeks later, the jihadist group seized the capital of Anbar province after relentless waves of suicide bombings.

U.S. defense chief Ash Carter has blamed Ramadi’s fall mainly on Iraqi forces’ lack of will to fight. But Islamic State’s battlefield performance suggests the terrorist group’s tactical sophistication is growing—a development the Iraqis and the U.S.-led coalition have so far failed to counter, said Iraqi officials, former U.S. officials and military analysts studying the organization. […]