Yahya Sinwar was given a hero's welcome in Gaza when he was freed in a 2011 prisoner exchange.
An Israeli official now describes that move as the "worst mistake in Israel's history."
Since then, Sinwar has vowed to fight for the release of all remaining Palestinians being held in Israeli jails — currently estimated to be some 7,000 people.
Sinwar grew up in the Khan Younis refugee camp and was detained by Israel in the late 1980s.
During interrogations, it emerged that he was involved in the killing of a dozen Palestinians suspected of collaborating with Israel.
Former Shin Beit security official Michael Koubi, who spent more than a hundred hours interrogating Sinwar, described him as both rational — and ruthless:
(Michael Koubi, former Israeli interrogator)
"Yahya Sinwar ruled at the prison, he was the chief of all Hamas prisoners. He told me, 'the Hamas is my wife, the Hamas is my child, the Hamas for me is everything. I'll make everything to succeed as a Hamas officer. And be sure that I'm going to kill a lot, a lot of Israelis, in Gaza or outside Gaza."
Sinwar used his time in prison to learn Hebrew and study his captors. He became convinced that Israel could be pressured into releasing large numbers of Palestinian prisoners in exchange for Israeli hostages.
After Sinwar himself was released, he capitalized on the networks he'd built in prison to become Hamas's chief in Gaza, alongside leader Ismail Haniyeh.
When Sinwar was reconfirmed in the post in 2021, he again pledged to get all Palestinians freed from Israeli jails.
While some in Israel may have believed Sinwar was a pragmatist who could be worked with, behind the scenes Hamas was plotting attacks on an unprecedented scale.
As Hamas presses for the release of imprisoned operatives, the miscalculations surrounding Yahya Sinwar will be a powerful reminder to Israel of the risk of making a wrong decision.