The Obama administration announced Wednesday the deployment of up to 450 additional military advisers to Iraq, deepening U.S. involvement in the war against the Islamic State and underscoring the fragility of a U.S. strategy that rests on the abilities of local forces.

The new troops, once in place, will bring the number of American service members in Iraq to around 3,500. The soldiers, who will be stationed at Taqaddum, an air base near the militant-held city of Ramadi, are expected to be focused on assisting Iraqi forces and tribal fighters prepare for an offensive to reclaim the western city, which fell to Islamic State militants in May following a retreat by Iraqi forces.

“This train, advise, and assist mission builds on lessons learned during the past several months,” the White House said in a statement. But recent months have revealed the deep-seated weaknesses of Iraqi troops, who abandoned their posts en masse a year ago in the northern city of Mosul. Since then they have made only halting progress against Islamic State militants, despite help from U.S. and allied airstrikes and Shiite paramilitary fighters who at times who have shouldered the bulk of the fighting. […]