The attack by Hezbollah highlights a weak spot in Israel’s vaunted air defense system that military officials say is hard to counter.

Israel has one of the world’s best defenses against missiles and rockets, but struggles to detect slower-moving unmanned aircraft, experts said.

TEL AVIV, Israel — One of the worst mass casualty strikes on Israel in a year of war came not from dozens of Iranian ballistic missiles nor the repeated barrages of rocket fire launched by Hamas and Hezbollah. Instead, it was a single drone.