When I read the news that John Brennan was set to appear before the Senate in hopes of becoming of the C.I.A. director, I thought of the group of villagers I met at a seaside hotel in Yemen two years ago. They had driven many miles to see me, coming from the Yemen countryside in a pair of battered taxis, and they were waiting in the hotel parking lot. There were about a dozen of them in all. It was a beautiful hotel, called the Mercure, with panoramic views of Aden harbor. The villagers, dressed in robes and rags, looked out of place, but they’d come to talk.

I had flown to Yemen to report on the popular uprising that was unfolding against the regime of Ali Abdullah Saleh, but I was also trying to find out about the secret war that the United States was waging there. In December, 2009, the Yemeni government had announced that its Air Force had bombed an Al Qaeda training camp in the village of Al Majalah, in a remote corner of the country, killing thirty-four fighters, and that the U.S. had provided the intelligence for the strike. The reality, as I discovered, was different.

For starters, as American officials confirmed, the attack was not carried out by the Yemeni Air Force but, rather, by the United States. The U.S. had launched a volley of Tomahawk cruise missiles from a ship off the coast. (As far as we know, most of the attacks in Yemen since then have been carried out with drones.) As was later revealed in documents released by Wikileaks, American and Yemeni officials had reached a secret agreement that allowed the U.S. to take action against suspected terrorists. The Yemeni President told General David Petraeus, then the head of CENTCOM, “We’ll continue saying the bombs are ours, not yours.”

As I wrote in a Letter from Yemen, in 2011, the villagers from Al Majalah had come to the hotel parking lot to tell me their story:

Hussein Abdullah, a herdsman, told me that he had been tending a herd of goats and camels when Al Majalah was hit. He recalled lying in his tent at sunrise, half-awake, when there was an enormous flash. “The sky turned white,” Abdullah said. “Everything suddenly disappeared.” He was knocked unconscious, and when he came to, he told me, he saw his wife running toward him. “And when she threw her arms around me I felt blood all over me,” he said. She died, as did his daughter; only his infant son survived.

That same evening, I met a fifteen-year-old girl named Fatima Ali, who, when she rolled up the sleeves of her chador, showed me terrible burns. Another girl was missing a finger. Her mother, she said, had been killed by the strike.

Some months after the attack in Al Majalah, Amnesty International released photos showing an American cluster bomb and a propulsion unit from a Tomahawk cruise missile. A subsequent inquiry by the Yemeni parliament found that fourteen Al Qaeda fighters had been killed—along with forty-one civilians, including twenty-three children.

Later, when I spoke to American officials, they seemed genuinely perplexed. They didn’t deny that a large number of civilians had been killed. They felt bad about it. But the aerial surveillance, they said, had clearly showed that a training camp for militants was operating there. “It was a terrible outcome,” an American official told me. “Nobody wanted that.”

None of the above is intended as an attack on Brennan, who has spent the past four years as President Obama’s counterterrorism advisor. He has a hard job. He is almost always forced to act on the basis of incomplete information. His job is to keep Americans safe, and he’s done that. Al Qaeda’s leadership, particularly in the tribal areas of Pakistan, has been decimated. Operating in Yemen, where vast tracts of the country lie beyond anyone’s control, cannot be easy.

But, as the details from the Al Majalah show, even the best-intentioned public servants operating with what appears to be decent intelligence can get things horribly wrong. Maybe Al Majalah was indeed an Al Qaeda training camp—maybe those aerial surveillance images were spot on. But, in retrospect, we know that the cameras missed the women and children.

Indeed, if there is one overriding factor in America’s secret wars—especially in its drone campaign—it’s that the U.S. is operating in an information black hole. Our ignorance is not total, but our information is nowhere near adequate. When an employee of the C.I.A. fires a missile from an unmanned drone into a compound along the Afghan-Pakistani border, he almost certainly doesn’t know for sure whom he’s shooting at. Most drone strikes in Pakistan, as an American official explained to me during my visit there in 2011, are what are known as “signature strikes.” That is, the C.I.A. is shooting at a target that matches a pattern of behavior that they’ve deemed suspicious. Often, they get it right and they kill the bad guys. Sometimes, they get it wrong. When Brennan said, as he did in 2011—clearly referring to the drone campaign—that “there hasn’t been a single collateral death,” he was most certainly wrong.

The same is true of opponents of the drone war, who sometimes lay claim to much more knowledge than they actually possess. And so, when a Pakistani newspaper reports that twenty civilians were killed in an attack, it is often taken as gospel truth, even though, as is often the case, the reporting is done over the telephone. For Americans—who are, after all, the ones whose country is firing the drones—it’s more or less impossible to independently verify many details of a drone strike. The reason is obvious: for a Western diplomat or reporter to go to the area where most of the drone strikes have taken place would be reckless in the extreme. (I’ve been to the tribal areas twice on my own. The first time, I was arrested and expelled by the Pakistani government; the second time, I was invited by a Taliban warlord who was killed six weeks later. Each trip took days of preparation and negotiation to arrange.)

The best and most painstaking attempts to get at the truth of the drone war—like one by the New America foundation—acknowledge the difficulty of the enterprise. The New America study found that between 2004 and 2010, the U.S. carried out a hundred and fourteen strikes, which the study’s authors estimated killed between eight hundred and thirty and twelve hundred and ten people. Of those, the study found, between five hundred and fifty and eight hundred and fifty—roughly two-thirds—were probably militants. Included in the dead were many militant leaders. That means that roughly a third of the dead—several hundred—were probably civilians. That’s a lot of bodies. These may be the best estimates we have, but they are still approximations.

Brennan is likely to face sharp questioning in front of the Senate Intelligence Committee, as well he should. You will hear a lot of claims about militants killed and civilians killed and civilians spared. Most likely, neither side will be entitled to its shrillness. If the Al Majalah strike has any value now, it should be to remind us not just of our knowledge but also of our ignorance.

Photograph by U.S. Air Force/Reuters.