The words “never again” ring hollow as the city of Aleppo, Syria, has fallen to regime forces of Bashar Assad. A brutal siege that has ground on for years was finally brought to a bloody end by a surge of Russian airpower, Iranian shock troops and assorted regional militia fighters. As we eulogize the dead of Aleppo, we must acknowledge the United States’ complicity in this tragedy.

President Barack Obama speaks of the need to “bear witness” to injustice. He did little else for Aleppo. To what have we borne witness? To the use of smart bombs to target women and children, hospitals and bakeries, aid warehouses and humanitarian convoys. To the development and popularization of barrel bombs — oil drums packed with shrapnel and explosives, dropped indiscriminately from aircraft to kill and maim as many civilians as possible. To the tactic of follow-on airstrikes designed to kill rescue workers, such as the intrepid White Helmets, who rush to the scene of an attack to save the innocent. And now to the busloads of refugees pouring out of Aleppo and the tens of thousands left behind to the tender mercies of the Assad regime and its Russian and Iranian allies.

Obama has borne witness to all of this, and more, and done nothing to stop it.

As with past atrocities, Aleppo’s destruction inspired much high-minded talk and the illusion of action. Endless meetings in the gilded palaces of Geneva and Vienna and elsewhere. Red lines drawn and transgressed with no consequences. Statements like this: “Should we really accept the notion that the world is powerless in the face of a Rwanda, or Srebrenica?” the president asked the U.N. General Assembly in 2013. “If that’s the world that people want to live in, they should say so, and reckon with the cold logic of mass graves.”

That reckoning is now upon us. The mass graves are before us, and the name Aleppo will echo through history, like Srebrenica and Rwanda, as a testament to our moral failure and everlasting shame. Even in a conflict that has killed nearly 500,000 people, driven half of Syria’s population from their homes, created the worst refugee crisis in Europe since World War II and spawned the terrorist army of the Islamic State — even amid all this horror and depravity, Aleppo stands out.

We must acknowledge that we have a stake in what happens in Syria. It is not just about the suffering of others, as moving as that is. It is about the national security of the United States: The resurgence of al-Qaida in Syria affects us. The rise of the world’s most advanced terrorist organization affects us, as we saw in Paris and San Bernardino, Calif. A refugee crisis that destabilizes allies such as Israel and Jordan and threatens the foundation of Western democracies affects us.

We must also acknowledge that Assad, Russian President Vladimir Putin and Qasem Soleimani, commander of Iran’s Quds Force, will never be viable counterterrorism partners. In fact, the opposite is true. The Syrian regime, Russia and Iran are not fighting the Islamic State. Their indiscriminate slaughter of Syrian civilians is what created the conditions for the Islamic State’s emergence. The bloody siege of Aleppo will be a windfall for terrorist radicalization and recruitment. To think that we can destroy the Islamic State by throwing in our lot with those who are strengthening it every day is a dangerous fantasy.

Finally, we must acknowledge that ending the conflict in Syria will not be possible until Assad and his foreign backers realize they cannot succeed militarily. And make no mistake: Succeeding militarily is what they are trying to do. The fall of Aleppo will only encourage them to turn their guns on their next targets in Syria. We must recall the wisdom of former secretary of state George Shultz: “Diplomacy not backed by strength will always be ineffectual at best, dangerous at worst.”

Just because America cannot stop every horror in the world does not absolve us of the responsibility of using our great power to end the worst injustices where we can. We cannot wall ourselves off from the chaos of our dangerous world. And if we try, the instability, terror and destruction at the heart of that chaos will eventually make their way to our shores.

John McCain, a Republican, represents Arizona in the U.S. Senate.