Our era is known as the Age of Terror, and no wonder. Twelve years ago, the United States suffered its worst terrorist attack ever, and since then, we have lived under the shadow of atrocities designed to frighten as well as kill. The bombs that went off in Boston put to rest the hope that with al-Qaida largely demolished, we could rest easy.

This episode was a gruesome reminder of the toxicity of political extremism and our vulnerability to it. There are people out there who want to kill innocent Americans, and there is no reliable way to stop them all. The question looking back is: Who did it? The question looking forward is: What’s next?