Yahya Sinwar, the Palestinian militant leader who emerged from two decades of prison in Israel to rise to the helm of Hamas and help plot the deadliest assault on Israel in its history, died on Thursday. He was in his early 60s.

A longtime Hamas leader who assumed its top political office in August, Mr. Sinwar was known among supporters and enemies alike for combining cunning and brutality. He built Hamas’s ability to harm Israel in service of the group’s long-term goal of destroying the Jewish state and building an Islamist, Palestinian nation in its place.

He played a central role in planning the surprise assault on southern Israel on Oct. 7, 2023, that killed about 1,200 people, brought 250 others back to Gaza as hostages and put him at the top of Israel’s kill list. Israeli leaders vowed to hunt him down, and the military dropped fliers over Gaza offering a $400,000 reward for information on his whereabouts.

But for more than a year, he remained elusive, surviving in tunnels Hamas had dug beneath Gaza, even as Israel killed many of his fighters and associates.

Mr. Sinwar’s legacy among Palestinians is complex. He built a force capable of striking the Middle East’s most sophisticated military despite the tight Israeli-Egyptian blockade of Gaza. But the Oct. 7 attack led Israel to commit not just to ending Hamas’s 17-year rule of Gaza, but also to destroying the group altogether.

The assault raised Hamas’s standing in the Israeli-occupied West Bank and elsewhere in the Arab world, according to polls, but not among Gazans, whose lives and homes bore the brunt of Israel’s subsequent invasion.