Yahya Sinwar Helped Start the Gaza War. Now He’s Key to Its Endgame.

After Hamas attacked Israel in October, igniting the Gaza war, 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.