The advance of Islamic militants across Iraq has brought fresh criticism for the Obama administration — but may also deliver a grim measure of vindication to one very prominent White House official: Vice President Joe Biden. In recent months, former officials and pundits questioned and even ridiculed Biden’s foreign policy acumen. Former Defense Secretary Bob Gates wrote in his memoir that Biden “was wrong on nearly every major foreign policy and national security issue of the past four decades.” And former Secretary of State Hillary Clinton’s new book “Hard Choices” notes that Biden “remained skeptical” about launching the raid that...