CAIRO — Hezbollah leader Hasan Nasrallah blamed Israel on Thursday for the explosion of electronic devices across Lebanon this week, calling the operation an “act of war” and vowing to respond, as the Israeli military signaled it was moving ahead with plans for a new phase of the conflict along the two countries’ border. “It is an act of war against the people of Lebanon, Lebanon’s sovereignty. It is a declaration of war,” Nasrallah said in a televised address, his first remarks since pagers...