An examination of how Ramadi fell indicates that Islamic State commanders executed a complex battle plan that outwitted a greater force of Iraqi troops as well as the much-lauded, U.S.-trained special-operations force known as the Golden Division, which had been fighting for months to defend the city.

Islamic State commanders evaded surveillance and airstrikes to bring reinforcements to its front lines in western Iraq. The group displayed a high degree of operational security by silencing its social media and propaganda teams during the Ramadi surge.

The group also churned out dozens of formidable new weapons by converting captured U.S. military armored vehicles designed to be impervious to small-arms fire into megabombs with payloads equal to the force of the Oklahoma City bombing.

Over the three-day surge in Ramadi, Islamic State fighters launched at least 27 such vehicle-borne improvised explosive devices, or Vbieds, that destroyed Iraq security forces’ defensive perimeters and crumbled multistory buildings.