There are at least two ways for mutual deterrence between states—also known as mutually assured destruction—to come to an end. As the Cold War taught us, one side in the conflict can simply collapse. But deterrence can also break down when one party decides to upend the equilibrium. For more than nine months after Hamas’s horrific October 7 attack on southern Israel and the start of Israel’s devastating onslaught in Gaza, hostilities between Israel and the Iran-led “axis of resistance” (the alliance that also includes Hezbollah, Syria, Iraqi paramilitary groups, and Ansar Allah—commonly known as the Houthis—in Yemen) built steadily. The fighting between Israel and Hezbollah in particular defied both sides’ previous red lines. Yet they appeared to steer clear of provoking all-out war. Then, in late July, Israel attacked a residential guesthouse in Tehran, killing Ismail Haniyeh, the political leader of Hamas. Just hours earlier he had attended the inauguration of Iran’s new president, Masoud Pezeshkian. For Israel to assassinate Haniyeh while he was staying in the Iranian capital as an official state guest …