In the immediate days after the deadly Oct. 7 Hamas attacks on Israel, Israeli intelligence officials feared a pre-emptive strike was imminent from another longtime enemy, Hezbollah. They frantically prepared to stop it with plans to strike and kill Hassan Nasrallah, the powerful Hezbollah leader who the Israelis knew would be in a bunker in Beirut.

But when Israel informed the White House of its plans, alarmed administration officials discounted the imminent Hezbollah strike. President Biden called Prime Minister Benjamin Netanyahu, told him that killing Mr. Nasrallah would set off a regional war and asked him to hold his fire, current and former senior American and Israeli officials said.

On Saturday, Israel announced that it had killed Mr. Nasrallah after warplanes dropped more than 80 bombs on four apartment buildings in Lebanon, where the Hezbollah leader of more than three decades had gone to meet his top lieutenants. Mr. Biden was not informed ahead of time, aggravating the White House.

But the more salient outcome for both Israel and the United States was how successfully Israeli intelligence had pinpointed Mr. Nasrallah’s location and penetrated Hezbollah’s inner circle. In a matter of weeks, Israel has decimated the senior and midlevel ranks of Hezbollah and left the group reeling.

That success is a direct result of the country’s decision to devote far more intelligence resources in targeting Hezbollah after its 2006 war with the Iran-backed terrorist group. It was a defining moment for Israeli intelligence. The Israeli army and the intelligence agencies failed to score a decisive victory in that 34-day conflict, which ended with a U.N.-brokered cease-fire and allowed Hezbollah, despite heavy losses, to regroup and prepare for the next war with Israel.