Hamas’s leader in Gaza is considered an architect of the Oct. 7 attacks that prompted Israel to retaliate. As mediators seek a cease-fire, a deal depends on Mr. Sinwar as well as his Israeli foes.

After Hamas attacked Israel in October, igniting the war in Gaza, Israeli leaders described the group’s most senior official in the territory, Yahya Sinwar, as a “dead man walking.” Considering him an architect of the raid, Israel has portrayed Mr. Sinwar’s assassination as a major goal of its devastating counterattack.

Seven months later, Mr. Sinwar’s survival is emblematic of the failures of Israel’s war, which has ravaged much of Gaza but left Hamas’s top leadership largely intact and failed to free most of the captives taken during the October attack.

Even as Israeli officials seek his killing, they have been forced to negotiate with him, albeit indirectly, to free the remaining hostages. Mr. Sinwar has emerged not only as a strong-willed commander but as a shrewd negotiator who has staved off an Israeli battlefield victory while engaging Israeli envoys at the negotiating table, according to officials from Hamas, Israel and the United States. Some spoke on the condition of anonymity to discuss sensitive intelligence assessments of Mr. Sinwar and diplomatic negotiations.

While the talks are mediated in Egypt and Qatar, it is Mr. Sinwar — believed to be hiding in a tunnel network beneath Gaza — whose consent is required by Hamas’s negotiators before they agree to any concessions, according to some of those officials.

Hamas officials insist that Mr. Sinwar does not have the final say in the group’s decisions. But though Mr. Sinwar does not technically have authority over the entire Hamas movement, his leadership role in Gaza and his forceful personality have given him outsize importance in how Hamas operates, according to allies and foes alike.

“There’s no decision that can be made without consulting Sinwar,” said Salah al-Din al-Awawdeh, a Hamas member and political analyst who befriended Mr. Sinwar while they were both jailed in Israel during the 1990s and 2000s. “Sinwar isn’t an ordinary leader, he’s a powerful person and an architect of events. He’s not some sort of manager or director, he’s a leader,” Mr. al-Awawdeh added.

Mr. Sinwar has rarely been heard from since the start of the war, unlike Hamas officials based outside Gaza, including Ismail Haniyeh, the movement’s most senior civilian official. Though he is nominally junior to Mr. Haniyeh, Mr. Sinwar has been central to Hamas’s behind-the-scenes decision to hold out for a permanent cease-fire, American and Israeli officials say.

Waiting for Mr. Sinwar’s approval has often slowed the negotiations, according to officials and analysts. Israeli strikes have damaged much of Gaza’s communications infrastructure, and it has sometimes taken a day to get a message to Mr. Sinwar and a day to receive a response, according to U.S. officials and Hamas members.

For Israeli and Western officials, Mr. Sinwar has over the course of these negotiations, which stalled again in Cairo this past week, emerged as both a brutal adversary and a deft political operator, capable of analyzing Israeli society and appearing to adapt his policies accordingly.

As an architect of the Oct. 7 attacks, Mr. Sinwar masterminded a strategy that he knew would provoke a ferocious Israeli response. But in Hamas’s calculus, the deaths of many Palestinian civilians — who do not have access to Hamas’s subterranean tunnels — were the necessary cost of upending the status quo with Israel.

American and Israeli intelligence agencies have spent months assessing Mr. Sinwar’s motivations, according to people briefed on the intelligence. Analysts in both the United States and Israel believe that Mr. Sinwar is primarily motivated by a desire to take revenge on Israel and weaken it. The well-being of the Palestinian people or the establishment of a Palestinian state, the intelligence analysts say, appears to be secondary.

An Understanding of Israeli Society

Mr. Sinwar was born in Gaza in 1962 to a family that had fled its home, along with several hundred thousand other Palestinian Arabs who fled or were forced to flee during the wars surrounding the creation of the state of Israel.

Mr. Sinwar joined Hamas in the 1980s. He was later imprisoned for murdering Palestinians whom he accused of apostasy or collaborating with Israel, according to Israeli court records from 1989. Mr. Sinwar spent more than two decades in Israeli detention before being released in 2011, along with more than 1,000 other Palestinians, in exchange for one Israeli soldier captured by Hamas. Six years later, Mr. Sinwar was elected leader of Hamas in Gaza.

While in prison, Mr. Sinwar learned Hebrew and developed an understanding of Israeli culture and society, according to fellow former inmates and Israeli officials who monitored him in prison. Mr. Sinwar now appears to be using that knowledge to sow divisions in Israeli society and heighten pressure on Benjamin Netanyahu, the Israeli prime minister, according to Israeli and U.S. officials.

They believe that Mr. Sinwar has timed the release of videos of some Israeli hostages in order to spur public outrage at Mr. Netanyahu during crucial phases of the cease-fire talks.

Some Israelis want the remaining hostages released even if it means agreeing to Hamas’s demands for a permanent truce that would keep the group — and Mr. Sinwar — in power. But Mr. Netanyahu has been reluctant to agree to end the war, partly because of pressure from some of his right-wing allies, who have threatened to resign if the war concludes with Hamas unbroken.

If Mr. Netanyahu has been accused of dragging out the fighting for personal benefit, so, too, has his archenemy, Mr. Sinwar.

Israeli and U.S. intelligence officers say that Mr. Sinwar’s strategy is to keep the war going for as long as it takes to shred Israel’s international reputation and to damage its relationship with its primary ally, the United States. As Israel faced intense pressure to avoid launching an operation in Rafah, Hamas fired rockets last Sunday from Rafah toward a nearby border crossing, killing four Israeli soldiers.

If this was a gambit by Hamas, it appeared to pay off: Israel began an operation this past week on the fringes of Rafah, and against that backdrop President Biden made his strongest criticism of Israeli policy since the war began. Mr. Biden said he would halt some future arms shipments if the Israeli military began a full-scale invasion of the city’s urban core.

U.S. officials say Mr. Sinwar is most likely in the tunnels under Khan Younis, the next major city to the north — intelligence that could undercut the Israeli rationale for the military operations in Rafah.

Projecting an Image of Unity

Hamas and its allies deny that either Mr. Sinwar or the movement is trying to leverage further Palestinian suffering.

“Hamas’s strategy is to stop the war right now,” said Ahmed Yousef, a Hamas veteran based in Rafah. “To stop the genocide and the killing of the Palestinian people.”

U.S. officials say that Mr. Sinwar has shown disdain for his colleagues outside Gaza, who were not informed about the precise plans for Hamas’s attack on Oct. 7. American officials also believe that Mr. Sinwar approves military operations conducted by Hamas, though Israeli intelligence officers say they are unsure of the extent of his involvement.

A senior Western official familiar with the cease-fire negotiations believes that Mr. Sinwar appears to makes decisions in concert with his brother, Muhammad, a senior Hamas military leader, and that throughout the war he had sometimes disagreed with Hamas leaders outside Gaza. While the outside leadership has at times been more willing to compromise, Mr. Sinwar is less ready to concede ground to the Israeli negotiators, in part, because he knows that he is likely to be killed whether or not the war ends, the official said.

Even if negotiators seal a cease-fire deal, Israel is likely to pursue Mr. Sinwar for the rest of his life, the official said.

Hamas members have projected an image of unity, downplaying Mr. Sinwar’s personal role in decision-making and maintaining that Hamas’s elected leadership collectively determines the movement’s trajectory.

Some say that if Mr. Sinwar has played a bigger role during this war, it is mostly because of his position: As the leader of Hamas in Gaza, Mr. Sinwar has greater say, though not the final call, according to Mousa Abu Marzouk, a senior Hamas official based in Qatar.

“Sinwar’s opinion is very important because he’s on the ground and he’s leading the movement on the inside,” said Mr. Abu Marzouk, the first leader of Hamas’s political office in the 1990s.

But Mr. Haniyeh has the “final say” on key decisions, Mr. Abu Marzouk said, adding that all of Hamas’s political leaders were of “one opinion.” Mr. Haniyeh could not immediately be reached for comment.

Still, there is something unusual about Mr. Sinwar’s force of personality, according to Mr. al-Awawdeh, his friend from prison. Other leaders might not have instigated the Oct. 7 attack, preferring to focus on technocratic matters of governance, Mr. al-Awawdeh said.

“If someone else had been in his position, things might have gone in a calmer way,” he said.

Mr. Sinwar himself could not be reached for comment and has rarely been heard from since October. U.S. and Israeli officials have said Mr. Sinwar is hiding near hostages, using them as human shields. An Israeli hostage who was released during a truce in November said she met Mr. Sinwar during her captivity.

In February, the Israeli military published a video that it said soldiers had taken from a security camera they found in a Hamas tunnel beneath Gaza. The video showed a man hurrying down the tunnel, accompanied by a woman and children.

The military said the man was Mr. Sinwar, fleeing with his family.

The claim was impossible to verify: The man’s face was turned away from the camera.[[

Excerpts: New York Times