Yahya Sinwar, the leader of Hamas in Gaza, is emerging as the mastermind behind the recent assault on Israel. Sinwar had previously expressed a desire for a ceasefire and portrayed Hamas as focused on governance rather than violence. Israeli officials now acknowledge that they had become complacent about the threat posed by Hamas. Sinwar, who was imprisoned by Israel for years, became fluent in Hebrew and closely studied Israeli society. He was released in 2011 and quickly returned to his connections and activities within Hamas.