The death of Yahya Sinwarlucky time, the Hamas leader who was believed to be the mastermind of the Oct. 7 attack, provides a new opening for the United States to meaningfully push for a cease-fire and hostage release in Gaza and for the de-escalation of violence across the Middle East.

While Mr. Sinwar was far from alone in resisting an agreement — Prime Minister Benjamin Netanyahu of Israel has repeatedly and openly undermined cease-fire efforts for months — his death can and must create new momentum to end this catastrophic and steadily widening war.

Born in the Khan Younis refugee camp in Gaza in 1962, Mr. Sinwar turned to militant activism in the early 1980s, and was imprisoned at least three times by Israel in that decade. He was sentenced to multiple life terms for the murder of two Israeli soldiers and four Palestinians suspected of collaborating with Israel, and released from prison in 2011 as part of the prisoner swap for the Israeli soldier Gilad Shalit.

Mr. Sinwar became Hamas’s leader in Gaza in 2017 and, with the Oct. 7 attacks, clearly hoped to set off a wider war against Israel. In the months that followed, while he was known to support an agreement for a permanent end to the war, he steadfastly refused the imposition of any new Israeli conditions.

A majority of Hamas’s senior leadership now resides outside of Gaza, mostly in Doha, Qatar, making it potentially easier to strike a deal. But for such a deal to be durable, it would need to really end the war, not simply start a new chapter of an Israeli military presence in Gaza.

If Mr. Sinwar truly was the obstacle to a cease-fire agreement that U.S. officials — including President Biden — have claimed, that obstacle is now gone. The United States and its partners have a window to halt the downward spiral to regional conflagration. The Biden administration must press the Netanyahu government and remaining Hamas officials to end the war in Gaza, return hostages to their families, surge humanitarian aid into the territory and urgently take other steps to ensure that Gazans have adequate shelter, supplies and security as winter approaches.

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