WASHINGTON—Army chief of staff Gen. Ray Odierno issued the marching orders in the fall of 2013. Some of the Army’s brightest officers would draft an unvarnished history of its performance in the Iraq War.

A towering officer who served 55 months in Iraq, Gen. Odierno told the team the Army hadn’t produced a proper study of its role in the Vietnam War and had to spend the first years in Iraq relearning lessons. This time, he said, the team would research before memories faded and publish a history while the lessons were most relevant.

It would be unclassified, he said, to stimulate discussion about the intervention—one that deepened the U.S.’s Mideast role and cost more than 4,400 American lives. He arranged for 30,000 pages of documents to be declassified. For nearly three years, the team studied those papers and conducted more than 100 interviews.

By June 2016, it had drafted a two-volume history of more than 1,300 pages. H.R. McMaster, the former national security adviser to President Trump, reviewed the tomes while a three-star general. He said in an interview last month it was “by far the best and most comprehensive operational study of the U.S. experience in Iraq between 2003 and 2011.”

The study’s title: “The United States Army in the Iraq War.”