The assassination of the Hamas political leader Ismail Haniyeh in Tehran on Tuesday — presumably carried out by Israel — has likely halted Gaza cease-fire talks and a hostage deal for the time being. It has also brought the region one step closer to an all-out conflagration. Indeed, within hours, Iran’s supreme leader, Ayatollah Ali Khamenei, declared his intention to attack Israel.

The almost certain escalation from the Haniyeh assassination signals a fundamental flaw in President Biden’s Gaza policy: the hope that the Gaza war could be contained to Gaza. The possibility of regional conflict has always been Mr. Biden’s real red line. But for months, the war has already been spreading — to Yemen, Lebanon, Syria, Iraq and now, to Iran. The fact that it hasn’t yet erupted into even more widespread and intense conflict is the result of both diplomatic skill and a lot of luck, the latter of which appears to be running out.

Some in the U.S. foreign policy establishment argue that since neither the United States nor Iran desires full-scale war, cooler heads will prevail. But once uncorked, this kind of violence usually cannot be controlled. It’s important to understand that even if we are able to step back from the brink now, as we all must hope, this policy is both a moral and strategic failure, with consequences and costs in human lives, to U.S. credibility and to the so-called “rules-based order” we haven’t begun to comprehend.

The current precarious moment is the result of a series of false assumptions that U.S. policy was built on since well before the war began. On Oct. 6, the United States was heavily focused on stitching up an agreement between Israel and Saudi Arabia, premised in part on the idea that the Palestinian people could simply be caged in perpetuity, with a few upgrades here and there to the military occupation they have endured for nearly six decades, and a few nominal commitments to someday, maybe, end that occupation. The Oct. 7 attacks showed this to be a fantasy.

In the months after, the Biden administration delayed calls for a cease-fire, in the face of large-scale global and domestic protest and internal government dissent, while emboldening Israel’s right-wing government with both the sale of arms and political support. At the same time, regional conflict steadily spread.

Rocket fire from Lebanon began almost immediately after Oct. 7, driving tens of thousands of Israelis from their homes in the north and leaving some 60,000 internally displaced with no prospect of when they might return. Attacks by Yemen’s Houthi forces on shipping routes in the Red Sea imposed a burden on the global economy as freight costs more than doubled in January. Attacks by Iran-backed militias in Syria and Iraq on U.S. interests culminated in a drone attack on an American base in Jordan that killed three U.S. service members at the beginning of the year, to which the United States retaliated with strikes of its own.