Iraqi policemen walk during an airstrike against Islamic State militants in Mosul, Iraq, on March 4, 2017. Goran Tomasevic / Reuters

Three weeks ago, the U.S.-led coalition in Iraq and Syria appears to have carried out an airstrike in Mosul that killed upwards of 200 innocent civilians. Very sadly, I have enough experience with these kinds of incidents as a policymaker that I always want to wait to see the results of the U.S. military’s investigation before judging what happened. The U.S. military isn’t always correct in its conclusions, but at the very least, I want to see more data before making a judgement about the actions or intentions of those involved.

That is not, to say the least, what most people did.

The condemnations came swiftly, especially on social media. One of my favorite people on Twitter, Hend Amry, quickly and bluntly declared it murder. Ken Roth of Human Rights Watch suspected that Trump’s lax rules of engagement were a factor in the killings, even though the rules of engagement don’t seem to have actually changed. Amry and Roth were among the more restrained voices, though. Most comments I read were full of condemnatory outrage toward the U.S. military and the Trump administration, which is suspected of throwing caution to the wind in its efforts to hasten the demise of the Islamic State.

The reason I mention this is not to excuse whatever errors in judgment or execution may have led to the death of innocent civilians. I could go into greater detail about how the U.S. military and its allies prosecute targets. I could talk about the air tasking order, and dynamic targeting, and the nature of the fight in western Mosul in comparison to the rest of the campaign thus far. I could talk about the way in which the U.S. military avoids civilian casualties for not only moral reasons but also for strategic reasons—because killing the innocent creates more enemies than it eliminates. But tactics and strategy are not the focus of this.

The reason I mention the air strike in Mosul is because I want to talk about another air strike—one that didn’t happen.

This airstrike was in 2013, and it was the one the Obama administration did not authorize to enforce the famous “red line” against the use of chemical weapons by the Assad regime in Syria.

No one—not even President Obama, as far as I could tell—was satisfied with the Obama administration’s approach to the conflict in Syria. But if you assembled all of the Obama administration’s critics in one room, they would not agree on an obvious alternative. The problem is wicked enough to confound easy solutions, and each policy alternative had strategic and moral deficiencies.

If the critics can agree on one thing, though, they would agree that the Obama administration’s decision to not strike the Assad regime in 2013—before Russia had escalated its involvement—was a mistake and undermined international norms regarding the use of chemical weapons.

And although I had just left the administration when the president made his decision and would later return to serve another two years in the Pentagon, I would agree. It was a mistake.

But now I want to go back to Mosul, because the United States did eventually intervene forcefully in Syria and Iraq after the Islamic State took over wide swathes of territory. The United States did the right thing to save the Yezidis on Mount Sinjar, and to save the Kurds in both Syria and Iraq. The United States and its allies put steel in the spine of the Iraqis to defend Baghdad and to retake Ramadi, Fallujah, and now Mosul.

One complaint among the Iraqis and Kurds with whom we were fighting, naturally, was that we were too cautious with the way we used our air power. Rukmini Callimachi of The New York Times describes the frustration she heard from Kurdish fighters, before the apparently errant Mosul strike, about how the United States was too deliberate in the way in prosecuted targets from the air.

But as the Mosul strike demonstrated, there is no clean way to prosecute a war, no matter how many checks you put in place to ensure you’re only killing your armed adversaries. Even those checks themselves—and this was the complaint from our partners on the ground— might give the enemy more opportunities to prepare or to use human shields.

Once you make the determination to intervene, you lose some control. Clausewitz famously posited that war is comprised of rational, irrational, and non-rational forces. Policy makers and their generals design their plans in an effort to impose some logic on war as an enterprise. But the people themselves, as evidenced by Twitter, can be irrational in their passions. They can quickly sour on a conflict once it encounters those non-rational forces in war—what Clausewitz called the “friction” of war. “Everything in war is very simple,” Clausewitz lamented, “but the simplest thing is very hard.”

President Obama never served in combat but understood what Clausewitz meant: He understood that once a government takes a nation down the road to conflict, the government loses some control over what’s about to happen. The enemy gets a vote. Mistakes happen. Innocent people are killed. And the people, who may clamor for violence one moment, will turn on a conflict the moment something goes wrong.

It’s 2017, and there has been another horrifying chemical weapons attack in Syria. Some of the same people outraged by the Mosul airstrikes are now demanding a U.S.-led response. The Trump administration has already attempted to blame the Obama administration for what took place, but that will not fly: Donald Trump is the president now, and he—not President Obama—has responsibility for how the United States responds.

But this president would be wise to remember what his predecessor knew: War is a very imperfect instrument of policy. No man or woman has ever mastered it. And wise leaders carry their people into war’s endeavor with the understanding that the will of the people is fickle, and war will forever be as much a game of chance as skill.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.