JERUSALEM — Israel launched a series of intense airstrikes in southern Lebanon early Sunday in what it said was a pre-emptive strike against the Hezbollah militant group, threatening to trigger a broader regionwide war that could torpedo efforts to forge a cease-fire in Gaza.

The army said Hezbollah was planning to launch a heavy barrage of rockets and missiles toward Israel. The Iranian-backed group had been promising to retaliate for Israel’s assassination of a top commander late last…