When Islamic State fighters overran a string of Iraqi cities last year, analysts at United States Central Command wrote classified assessments for military intelligence officials and policy makers that documented the humiliating retreat of the Iraqi Army. But before the assessments were final, former intelligence officials said, the analysts’ superiors made significant changes. In the revised documents, the Iraqi Army had not retreated at all. The soldiers had simply “redeployed.†Such changes are at the heart of an expanding internal Pentagon investigation of Centcom, as Central Command is known, where analysts say that supervisors revised conclusions to mask some of...