We had no idea what we were doing.

Over the past few years, Americans have slowly come to grips with the notion that an invasion sold to us as a 60-day jaunt in the desert, with our troops welcomed as liberators and with Iraqis paying the cost for their country’s reconstruction, has instead cost us trillions of dollars and the lives of more than 4,000 Americans, with little to show for it.

Now we are confronted with an even greater potential tragedy: Now we watch, almost helpless, as the ambitions and emotions unleashed by our invasion morph into a broader Sunni-Shi’ite civil war that threatens to engulf not just Iraq but the entire Islamic world for years if not decades to come.

We had no idea. Arrogance and ignorance, it turns out, are a deadly combination. Even today, I’m not sure that most Americans understand the potential consequences of the tragedy playing out in the Middle East, a tragedy that we made all but inevitable with our decision to invade Iraq.

By invading Iraq, the keystone state of the Arab world, we destroyed the precarious balance in that country between Shi’ite and Sunni, and thus the balance in the larger Islamic world as well. Yes, that balance had been maintained by a cruel repression by Saddam Hussein, but with that repression removed we offered nothing to replace it.

In fact, the official position of the Bush administration was that there were no deep sectarian divides in Iraq, and thus no need to address them. We misread the situation in many, many ways, but that misreading was perhaps the most profound and consequential.

With everything in play, Shi’ites began killing Sunnis who began killing Shi’ites. Sunnis and Shi’ites outside Iraq were drawn into the battle. Weapons and money and fighters flowed from Shi’ite Iran and Sunni Saudi Arabia and other countries. The struggle engulfed Syria, and now Lebanon and Kuwait and Jordan are threatened.

Many of those who supported the invasion now try to claim that the war had been won with the surge of American troops in 2007, which succeeded in stopping the violence that then threatened to overwhelm Iraq. In a very narrow sense, they are right. The surge taught us that with 150,000 U.S. troops stationed in Iraq, actively engaged in combat and taking regular causalities, we could suppress the Sunni/Shi’ia civil war that elements of both sides were eager to fight.

But only fools would mistake that for a permanent solution. Everyone knew that the American people would not stand for a long-term commitment that large and that deadly, and neither would the Iraqis. Even at the time, President Bush made clear that the surge had merely won Iraqi leaders another opportunity to reconcile factions within their country, and that all would be lost if they didn’t seize their chance. They didn’t seem much interested in trying.

So now what? Whatever role we played in the past, the difficult truth is that the United States lacks the credibility or resources to resolve a religious war within Islam. The best we can accomplish may be to contain it. The even more difficult truth is that if a regional war erupts, U.S. troops may be called upon again, this time to mitigate what our own ignorance has ignited.