How will a new Hamas leader affect the war in Gaza and cease-fire efforts?

Yahya Sinwar’s appointment as the top leader of Hamas formalizes a role he assumed in the early hours of Oct. 7, when the surprise terrorist attack into Israel that he helped mastermind ushered in the bloodiest chapter of the Israeli-Palestinian conflict.

He is seen as a hard-liner with closer ties to Hamas’ armed wing than his predecessor, Ismail Haniyeh, who was killed in an explosion in Iran’s capital last month that was widely blamed on Israel.

Sinwar was already seen as having the final word on any cease-fire agreement for Gaza and the release of dozens of Israeli hostages still held by Hamas.

But he is deep in hiding inside Gaza, and mediators say it takes several days to exchange messages with him.

Hamas has survived the killing of several top leaders across more than three decades, while maintaining a high degree of internal cohesion — and tapping Sinwar, who tops Israel’s most-wanted list, was a show of defiance.

Sinwar has spent more than two decades in Israeli prisons and told interrogators he had killed 12 suspected Palestinian collaborators, gaining a reputation for brutality among people on both sides of the conflict.

He and Mohammed Deif, the shadowy head of Hamas’ armed wing who Israel claims to have killed in a recent strike, spent years building up the group’s military strength and are believed to have devised the Oct. 7 terrorist attack that killed some 1,200 people in Israel.

In recent negotiations, “Haniyeh had played a big role in trying to convince Sinwar to accept a cease-fire proposal with Israel,” said Hugh Lovatt, an expert on the Israeli-Palestinian conflict at the European Council on Foreign Relations.

Sinwar has stuck to demands for the release of hundreds of Palestinian prisoners, the full withdrawal of Israeli forces from Gaza and a lasting cease-fire.

Israeli Prime Minister Benjamin Netanyahu has vowed to continue the war until Hamas is completely destroyed and all the hostages return home.