Two years after US troops departed, Iraqi security forces are struggling to curb violence that has reached a level not seen since 2008, near the height of America’s military presence.

Iraqi forces are now alone in facing militant groups revitalized by widespread discontent among the minority Sunni Arab community, which complains of being marginalized and targeted by Shiite-led authorities, and by the brutal war in neighboring Syria.

They have shortcomings that experts say range from a decline in training and intelligence capabilities to politicization. […]