Kurdish Peshmerga fighters assemble at a shrine on Iraq's Mount Sinjar on Friday, December 19. The Kurdish military said that with the help of coalition airstrikes, it has "cleansed" the area of ISIS militants. ISIS has been advancing in Iraq and Syria as it seeks to create an Islamic caliphate in the region.
Editor's note: David Gergen is a senior political analyst for CNN and has been an adviser to four presidents. A graduate of Harvard Law School, he is a professor of public service and director of the Center for Public Leadership at Harvard University's Kennedy School of Government. Follow him on Twitter at @david_gergen. Daniel Katz, his research assistant, is a graduate of Brandeis University.The opinions expressed in this commentary are solely those of the authors.
(CNN) -- "How did it all begin?"
Exactly a year after the Cuban Missile Crisis, President John F. Kennedy stood before students and recalled how in 1914, Prince Bulow of Germany asked that question of the German Chancellor as Europe slid into a catastrophic world war. The chancellor wearily replied, "Ah, if one only knew."
Today, as we watch the fracturing of Iraq, we would be wise to ask a different question: "How does one end a war?" The answer remains elusive, but Iraq is quickly becoming a case study in how not to do it. And unless we learn from this experience, Afghanistan could well be next.
Unfortunately, as Gideon Rose shows in his 2010 book, "How Wars End," this is not the first time we've fumbled the end of a war. He argues that the U.S. has often gone wrong because it failed to leave a sustainable postwar structure in place, citing the Treaty of Versailles after World War I, a prolonged conflict and then stalemate in Korea, the chaos in Vietnam, and so on.
In this case, the administration that launched the Iraq war deserves the lion's share of blame for how badly it has gone. A case can be made that former President George W. Bush's team was misled by faulty intelligence on weapons of mass destruction, but they were the ones who made the wrong calls on going in with too few troops and then disbanding the Iraqi army, turning the country into a cauldron of sectarian hatreds and violence.
But after the 2007 troop surge, the Bush team could also make a credible argument they left an Iraq that was fairly stable and had a decent chance at self-governance. President Barack Obama himself proclaimed in 2011 as he withdrew the last American troops, "We're leaving behind a sovereign, stable and self-reliant Iraq."
So how did Iraq then tumble into a civil war? How come no top official in Washington saw it coming or appreciated how quickly Sunni jihadists would seize control of a third of the country? Could the U.S. have prevented the breakup?
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Speaking with military and national security leaders who have retired in the past few years, one finds a unanimous view that Iraqi Prime Minister Nouri al-Maliki was the one who blew it since American troops left him fully in charge. Forcing Sunnis and Kurds out of power, establishing an authoritarian, corrupt and often brutal Shia regime, and filling the ranks of his American trained army with cronies and incompetents, al-Maliki invited a rebellion. That's why recent U.S. military leaders think al-Maliki has to go -- immediately!
But these same leaders believe there was a chance -- no guarantees but a chance -- that if the United States had left behind a strong residual force under a status-of-forces agreement, the current disaster might have been avoided.
For one thing, a U.S. force on the ground would have sent a clear signal to insurgents not to mess with things. For another, the U.S. would have kept better intelligence and could more easily head off trouble. Just as important, a top U.S. general, speaking for the President, would have been whispering into al-Maliki's ear. U.S. commanders over the years -- from Petraeus to Odierno -- found this direct line of communication vital to keeping al-Maliki on a reform path and away from power consolidation.
The story of why we failed to leave behind a residual force has been well told elsewhere. One of the best sources is the 2013 book "The Endgame" by Michael Gordon and Gen. Bernard Trainor. This past week, Peter Beinart provided an excellent autopsy in a piece for The Atlantic. Beinart's is the more surprising as he attacks the Obama administration from a liberal vantage point.
Gordon and Trainor report that Obama's top military leadership -- Joint Chiefs Chairman Adm. Michael Mullen, Defense Secretary Robert Gates and others -- wanted to keep a residual force and thought anything less than 16,000 troops would be insufficient. But they ran into resistance on two fronts. The Prime Minister wanted Americans out; al-Maliki had designs of his own for the exercise of power. The Obama White House has long argued that it was al-Maliki who scuppered any deal.
As accounts like those of Gordon and Trainor along with Beinart have shown, however, the White House was always half-hearted about pushing. The President was skeptical of his military advisers and apparently sympathetic to political advisers who wanted to get out of Iraq pronto -- and certainly before the 2012 elections. So, Obama overruled recommendations from his military leaders and presented the Iraqis with a plan for 3,500 continuous troops. Squabbles broke out between both sides and eventually an agreement fell apart.
If Obama was unhappy, he had a strange way of showing it. In the run-up to elections in 2012, he traversed the campaign trail celebrating the fulfillment of his promise to pull every last troop out of Iraq.
We will never know for sure whether a residual American force would have saved Iraq as a sovereign, stable nation -- what it seemed when we left. Sectarian hatreds there run back centuries, and ultimately they must take responsibility for their own destinies. We can't save people who won't save themselves.
But it would not be surprising if in his next edition, Gideon Rose adds to his chapter on Iraq and repeats his argument that America's wars have often ended badly when we fail to put in place a sustainable postwar structure before we leave.
The looming question is whether we will now make the same mistake in Afghanistan. After all, the President has now declared that all American troops will be out of there in 2016 -- just by coincidence, of course, the same year as the next presidential election.
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A Peshmerga fighter looks down at the body of an alleged ISIS fighter in Zummar, Iraq, on Thursday, December 18.
A Kurdish fighter stands next to a destroyed armored vehicle in northern Iraq on December 18. The vehicle was destroyed by an improvised explosive device placed by ISIS militants.
Peshmerga fighters stop to check a vehicle in Zummar on December 18 as they continue to battle ISIS fighters near the border with Syria.
Pro-Iraqi government forces guard a shrine in Balad, Iraq, on Monday, December 15.
A Yazidi woman displaced by ISIS militants tends to a fire Wednesday, December 10, at a shelter in Dohuk, Iraq.
A Kurdish child from the Kobani, Syria, area holds laundry at a refugee camp in Suruc, Turkey, on Monday, November 17. Tens of thousands of people have fled Kobani, known in Arabic as Ayn al-Arab, to escape ISIS.
Smoke rises from Kobani following airstrikes on November 17. The United States and several Arab nations have been bombing ISIS targets to take out the group's ability to command, train and resupply its fighters.
A Kurdish child from the Kobani area holds on to a fence at a refugee camp in Suruc on Sunday, November 16.
People in Suruc watch smoke rise near the Syrian border during clashes between ISIS members and armed groups on Thursday, November 13.
A bomb (upper left) falls on an ISIS position in Kobani during an airstrike by the U.S.-led coalition on Saturday, November 8.
Iraqi military forces take up position in Jurf al-Sakhar, Iraq, on November 8.
Fire and smoke rise from Kobani following airstrikes against ISIS on Thursday, October 30.
Iraqi special forces search a house in Jurf al-Sakhar on October 30 after retaking the area from ISIS.
Smoke rises during fighting in Kobani on Monday, October 27.
ISIS militants stand near the site of an airstrike near the Turkey-Syria border on Thursday, October 23.
An explosion rocks Kobani during a reported car-bomb attack by ISIS militants on Monday, October 20.
People watch Kobani from a hill near the Turkey-Syria border on October 20.
Kurdish fighters walk to positions as they fight ISIS forces in Kobani on Sunday, October 19.
A U.S. Air Force plane flies above Kobani on Saturday, October 18.
Heavy smoke rises in Kobani following an airstrike by the U.S.-led coalition on Saturday, October 18.
Cundi Minaz, a female Kurdish fighter, is buried in a cemetery in the southeastern Turkish town of Suruc on Tuesday, October 14. Minaz was reportedly killed during clashes with ISIS militants in nearby Kobani.
Turkish police officers secure a basketball stadium in Suruc on October 14. Some Syrian Kurds have been held there since crossing from Syria into Turkey.
Kiymet Ergun, a Syrian Kurd, celebrates in Mursitpinar, Turkey, after an airstrike by the U.S.-led coalition in Kobani on Monday, October 13.
Smoke rises from Kobani on Sunday, October 12.
Syrian Kurds from Kobani stand outside the grounds of a refugee camp in Suruc on Saturday, October 11.
Alleged ISIS militants stand next to an ISIS flag atop a hill in Kobani on Monday, October 6.
In this photo released by the U.S. Air Force on Saturday, October 4, a U.S. Navy jet is refueled in Iraqi airspace after conducting an airstrike against ISIS militants.
A Kurdish Peshmerga soldier who was wounded in a battle with ISIS is wheeled to the Zakho Emergency Hospital in Duhuk on Tuesday, September 30.
Iraqi Shiite militiamen aim their weapons during clashes with ISIS militants in Jurf al-Sakhar on Sunday, September 28.
Syrian Kurds wait near a border crossing in Suruc as they wait to return to their homes in Kobani on Sunday, September 28.
Syrian Kurds wait behind border fences to cross into Suruc on September 28.
Tomahawk missiles, intended for ISIS targets in Syria, fly above the Persian Gulf after being fired by the USS Philippine Sea in this image released by the U.S. Navy on Tuesday, September 23.
Turkish Kurds clash with Turkish security forces during a protest near Suruc on Monday, September 22. According to 
Syrian Kurds fleeing ISIS militants wait behind a fence in Suruc on Sunday, September 21.
A elderly man is carried after crossing the Syria-Turkey border near Suruc on Saturday, September 20.
A Kurdish Peshmerga fighter launches mortar shells toward ISIS militants in Zummar on Monday, September 15.
An ISIS flag flies on the other side of a bridge at the front line of fighting between ISIS and Kurdish Peshmerga fighters in Rashad, Iraq, on Thursday, September 11.
U.S. Secretary of State John Kerry reads on a flight en route to Iraq on Wednesday, September 10. Kerry traveled to the Mideast to discuss ways to bolster the stability of the new Iraqi government and combat ISIS.
Kurdish Peshmerga fighters fire at ISIS militant positions from their position on the top of Mount Zardak, east of Mosul, Iraq, on Tuesday, September 9.
An Iraqi fighter jet flies over Amerli, Iraq, on September 3. Amerli had been under siege by ISIS militants.
Iraqi volunteer fighters celebrate breaking the Amerli siege on Monday, September 1. ISIS militants had surrounded Amerli, 70 miles north of Baquba, Iraq, since mid-June.
Kurdish Peshmerga forces stand guard at their position in the Omar Khaled village west of Mosul on Sunday, August 24.
Kurdish Peshmergas fight to regain control of the town of Celavle, in Iraq's Diyala province, on August 24.
Peshmerga fighters stand guard at Mosul Dam in northern Iraq on Thursday, August 21. With the help of U.S. military airstrikes, Kurdish and Iraqi forces
Displaced Iraqis receive clothes from a charity at a refugee camp near Feeshkhabour, Iraq, on Tuesday, August 19.
A fighter with Kurdish Peshmerga forces battles ISIS militants near Mosul on Monday, August 18.
Peshmerga fighters inspect the remains of a car that reportedly belonged to ISIS militants and was targeted by a U.S. airstrike in the village of Baqufa, north of Mosul, on August 18.
Kurdish Peshmerga fighters fire at ISIS in Khazair, Iraq, on Thursday, August 14.
Volunteers of the Iraqi Red Crescent Society unload boxes of goods before distributing them August 14 to families who fled from ISIS.
From the flight deck of the USS George H.W. Bush, which is in the Persian Gulf, two U.S. fighter jets take off for a mission in Iraq on Monday, August 11. U.S. President Barack Obama authorized airstrikes against Islamic militants and food drops for Iraqis who are trapped by the militants.
Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and some other Yazidi people are flown to safety Monday, August 11, after a dramatic rescue operation at Iraq's Mount Sinjar. A CNN crew was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. But only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.
Thousands of Yazidis are escorted to safety by Kurdish Peshmerga forces and a People's Protection Unit in Mosul on Saturday, August 9.
Iraqi Shiite volunteers who have joined government forces to fight ISIS take part in a training session near Basra, Iraq, on Thursday, August 7.
Thousands of Yazidi and Christian people flee Mosul on Wednesday, August 6, after the latest wave of ISIS advances.
A Baiji oil refinery burns after an alleged ISIS attack in northern Selahaddin, Iraq, on Thursday, July 31.
A Syrian rebel fighter lies on a stretcher at a makeshift hospital in Douma, Syria, on Wednesday, July 9. He was reportedly injured while fighting ISIS militants.
Iraqis who fled fighting in the cities of Mosul and Tal Afar try to enter a temporary displacement camp in Khazair on Wednesday, July 2.
Peshmerga fighters check cars at the entrance of a temporary displacement camp in Khazair on Thursday, June 26.
Kurdish Peshmerga take their positions behind a wall on the front line of the conflict with ISIS militants in Tuz Khormato, Iraq, on Wednesday, June 25.
Peshmerga fighters clean their weapons at a base in Tuz Khormato on June 25.
New army recruits gather in Najaf, Iraq, on Wednesday, June 18, following a call for Iraqis to take up arms against Islamic militant fighters.
An MH-60R Sea Hawk helicopter lands on the aircraft carrier USS George H.W. Bush in the Persian Gulf on Tuesday, June 17.
Newly recruited Iraqi volunteer fighters take part in a training session in Karbala, Iraq, on June 17.
Members of ISIS prepare to execute soldiers from Iraq's security forces in this image, one of many reportedly posted by the militant group online. CNN cannot independently confirm the authenticity of the images.
Iraqi men chant slogans outside of an army recruiting center to volunteer for military service Thursday, June 12, in Baghdad.
Kurdish Peshmerga forces, along with Iraqi special forces, deploy their troops and armored vehicles outside of Kirkuk, Iraq, on June 12.
Children stand next to a burnt vehicle during clashes between Iraqi security forces and ISIS militants in Mosul on Tuesday, June 10.
Civilians from Mosul escape to a refugee camp near Irbil, Iraq, on June 10.
