BAGHDAD, Iraq — When I retired from the Army in June, my comrades in arms laughed at my summer vacation plans: another August in Iraq.

But I had unfinished business here. When my unit left Iraq in September 2004, one of my battle captains presented us with coffee mugs. The sardonic inscription: "Operation Iraqi Freedom 2003-2004: We were winning when I left." We weren't, and we knew it. We had lost 22 men and more than 100 had been wounded during our tour. The decline of the U.S. project in Iraq over the following two years made it easy to believe their sacrifices had been for naught.

I've played the history over and over in my mind. Early mistakes by U.S. officials — disbanding the Iraqi army and firing almost all the civil servants needed to run a society — provided tinder for the Sunni insurgency. Things worsened as U.S. commanders withdrew our forces from the cities to large, comfortable bases from which they commuted to war. By the time my unit left, the Sunni insurgency that had erupted in Fallujah in 2003 had spread to the heart of Baghdad. Sunni and Shiite militias were pushing Iraq to the brink of full-scale civil war.

But studding this bleak narrative were signs of hope. As early as 2003, some of Iraq's minority Sunnis came to understand that their future lay not in resisting U.S. forces but in working with them. After neglecting Sunni tribes for years, our military began paying some to break ranks with the radicals of al-Qaida in Iraq, leading to the Anbar Awakening of late 2006.

Enter the "surge" and a new U.S. strategy: to protect Iraqis by living among them. Military units left the comfort of their bases and, with counterparts from the Iraqi army and police, manned the smaller encampments that dotted the country's cities and provinces. U.S. casualties also surged as we cleared Baghdad and surrounding communities in the summer of 2007.

Then, Iraqi Prime Minister Nouri al-Maliki moved against Shiite militias — first in Basra in March 2008, then in Sadr City, the massive Shiite neighborhood of Baghdad. Operations continue in Mosul and Diyala, and so far the success is undeniable.

Route Irish, which runs from Baghdad International Airport to the International Zone, used to be the most dangerous road in the world. When I drove down it last month, I saw flowers being planted in newly rebuilt medians. Guard rails — which we had to tear down to prevent IEDs from being hung behind them — are being reinstalled.

Walking through the Dora market in Baghdad, I felt naked without a weapon. But then I reminded myself that I was safe, protected by the Sons (and Daughters) of Iraq. I visited an athletic club started by a Dora entrepreneur with a micro-loan from my West Point classmate, Lt. Col. Tim Watson. The gym has a waiting list for membership.

Everyone I talked to, Iraqis and Americans alike, stressed that the security gains are fragile and reversible; there were two car bombs and a suicide vest attack in Mosul three days after our visit. But the improvements in Baghdad and Basra are striking, with increasingly competent Iraqi security forces on every street corner — although they will continue to need our advice and assistance for some years to come.

I am no cheerleader for the war in Iraq. We've made horrible mistakes that cost the lives of too many of my friends, American and Iraqi. It took us too long to learn from our errors and adopt an effective counterinsurgency strategy, and even now the war is far from won.

But the way ahead is becoming clearer, with a roadmap provided by men such as Col. Dominic Caracillo, a brigade commander in the 101st Airborne whose men patrol a patch south of Baghdad that used to be called the Triangle of Death. A year ago, there were as many as 50 attacks every week; now there are just a few. Caracillo is overseeing a drawdown of U.S. forces; his brigade of about 4,000 soldiers is shipping out, to be replaced by a task force of fewer than 1,000. This force will not conduct counterinsurgency operations, but will support the newly created 17th Division of the Iraqi army.

Iraq faces significant external security threats, as well as the remnants of al-Qaida in Iraq, Shiite militias and Sunni insurgent groups. But U.S. forces will increasingly be able to turn combat over to the Iraqis, allowing the United States to scale back its involvement significantly.

When I came home from Iraq in 2004, we weren't winning. Four years later, the situation has changed dramatically. I can scarcely bring myself to say it out loud, but this time, we were winning when I left.

John A. Nagl is a senior fellow at the Center for a New American Security and was an operations officer in Iraq in 2003 and 2004. He wrote this commentary for The Washington Post.