The chief participants in this debate—all Iraq War veterans—fixate on two large questions. First, why, after its promising start, did Operation Iraqi Freedom go so badly wrong? Second, how should the hard-earned lessons of Iraq inform future policy? Hovering in the background of this Iraq-centered debate is another war that none of the debaters experienced personally—namely, Vietnam.

The protagonists fall into two camps: Crusaders and Conservatives.

The Crusaders consist of officers who see the Army’s problems in Iraq as self-inflicted. According to members of this camp, things went awry because rigidly conventional senior commanders, determined “never again” to see the Army sucked into a Vietnam-like quagmire, had largely ignored unconventional warfare and were therefore prepared poorly for it. Typical of this generation is Lieutenant General Ricardo Sanchez, once the top U.S. commander in Baghdad, who in late 2003 was still describing the brewing insurgency as “strategically and operationally insignificant,” when the lowliest buck sergeant knew otherwise.

Younger officers critical of Sanchez are also committed to the slogan “Never again,” but with a different twist: never again should the officer corps fall prey to the willful amnesia to which the Army succumbed after Vietnam, when it turned its back on that war.

Among the Crusaders’ most influential members is Lieutenant Colonel John Nagl, a West Pointer and Rhodes Scholar with a doctorate from Oxford University. In 2002, he published a book, impeccably timed, titled Learning to Eat Soup With a Knife: Counterinsurgency Lessons From Malaya and Vietnam. After serving in Iraq as a battalion operations officer, Nagl helped rewrite the Army’s counterinsurgency manual and commanded the unit that prepares U.S. soldiers to train Iraqi security forces. (Earlier this year, he left the Army to accept a position with a Washington think tank.)

To Nagl, the lessons of the recent past are self-evident. The events of 9/11, he writes, “conclusively demonstrated that instability anywhere can be a real threat to the American people here at home.” For the foreseeable future, political conditions abroad rather than specific military threats will pose the greatest danger to the United States.

Instability creates ungoverned spaces in which violent anti-American radicals thrive. Yet if instability anywhere poses a threat, then ensuring the existence of stability everywhere—denying terrorists sanctuary in rogue or failed states—becomes a national-security imperative. Define the problem in these terms, and winning battles becomes less urgent than pacifying populations and establishing effective governance.

War in this context implies not only coercion but also social engineering. As Nagl puts it, the security challenges of the 21st century will require the U.S. military “not just to dominate land operations, but to change entire societies.”