The U.S.-led coalition announced last year that Iraq's army was capable of fighting IS remnants on its own and pulled out of eight bases across the country. Many of those withdrawals became permanent.
At the same time, citing the improving security situation, Baghdad's authorities lifted the concrete blast walls and checkpoints that had congested the city for years.
Battle-hardened units were moved out of cities to chase down IS sleeper cells in rural areas, with less-experienced units taking over urban security.
Security analyst Alex Mello said those rotations combined with less-reliable intelligence may have eventually granted IS ‘a gap to exploit.’
The US official said Iraqi forces were at times unwilling to tackle IS fighters head-on, allowing small cells to flourish into larger groups.
A part that is left after the greater part has been removed or destroyed.
The process or action of a military force moving out of an area.
Iraqi lawmakers called for a withdrawal of foreign forces.