Israel’s assassination of Hasan Nasrallah, Hezbollah’s longtime leader, left a shaken Lebanon on Saturday, a void the militant group might struggle to fill, and its battle with Israel on an uncertain and possibly more violent path.

Israel rejected global calls on Thursday for a ceasefire with the Hezbollah movement, defying its biggest ally the U.S. and pressing ahead with strikes that have killed hundreds in Lebanon and heightened fears of an all-out regional war.