In a display of military and intelligence prowess reminiscent of its surprise victory over Arab armies in the Six-Day War of 1967, Israel has delivered a series of devastating preemptive blows on Hezbollah, the Shiite Lebanese paramilitary force, culminating in the assassination of its longtime leader, Hasan Nasrallah, under a hail of bombs on Friday.

It’s a much-deserved comeuppance for an Iranian proxy militia, guilty of many terrorist deeds, which has held sway over Lebanon, assisted in the violent imposition of the Assad dynasty on Syria, and threatened Israel. As President Joe Biden noted Saturday, Mr. Nasrallah’s death brings “a measure of justice” for Hezbollah’s victims, who include not only Israelis and Jews far from Israel, but also 241 American and 58 French peacekeeping troops whom Hezbollah slaughtered in 1983 Beirut truck bombings.

Hezbollah’s sudden decapitation is also a disruptive geopolitical watershed. As such, it could either deepen the region’s crisis or mark the beginning of its end. Much depends on the wisdom with which governments, including the Biden administration, act in the next few days.

For decades, Hezbollah operated as an Iranian-sponsored state-within-a-state in Lebanon. But its power and its possession of a massive military stockpile lacked legitimacy, just as its unilateral rocketing of northern Israel last Oct. 8 had no rationale except to support Hamas’s ghastly attack and weaken Israel’s ability to fight back. Now, Hezbollah’s entire organization is tottering, confused and frightened by Israeli intelligence’s evidently thorough penetration of its ranks. And its Iranian-supplied arsenal, warehoused across Lebanon, stands at risk of destruction, which in turn could portend the reduction of the long-standing Hezbollah threat — useful to Iran — to overwhelm Israel with a missile barrage.

“Israel has momentum,” as Prime Minister Benjamin Netanyahu said in a short televised address after Mr. Nasrallah’s death. And he has a point: Nearly a year into what increasingly looked like a “forever war” against Hamas and other members of Iran’s regional “axis of resistance,” Israel has its enemies off balance.

The question for Mr. Netanyahu is: “Momentum toward what?” Israel has a history of short-run tactical military triumphs that proved strategically sterile — or even laid the basis for new conflicts. Examples include previous wars with Palestinian guerrillas or Hezbollah in Lebanon and, indeed, the 1967 war, whose legacy includes the occupation of the West Bank. Israel’s decapitation of Hezbollah, however impressive, comes unaccompanied, so far, by a plan for filling the political vacuum looming in Lebanon.

In addition to creating peace in northern Israel so that tens of thousands of displaced people can return to their homes, Israel’s goals seem to include forcibly delinking Hezbollah’s war from that of Hamas in Gaza. Though it had so far hesitated to launch the full arsenal of rockets in its possession, Hezbollah has sought to distract Israel’s military with a steady rain of projectiles at Israel, including a strike that cost the lives of 12 Druze children in July. The Lebanon-based group insisted that it would cease fire only when Israel and Hamas made a deal of their own.

Now there is a chance to change that. Much depends on Iran’s reaction. Tehran is weighing whether to go to war directly against Israel now that the proxies it depended on are reeling. The Biden administration’s task is to dissuade and deter Iran from taking that step up the escalatory ladder. Mr. Biden recognized as much by noting that he had instructed the Pentagon “to further enhance the defense posture of U.S. military forces in the Middle East region.”

In a best-case scenario, a display of American might, along with caution from Iran and a U.S. diplomatic push on Israel, could lay the basis for an end to hostilities, freedom for Hamas hostages in Gaza and a surge of humanitarian aid for Palestinians. Israel seems to prefer not to have to follow up its air campaign by going into Lebanon on the ground, which would be costly for both the Jewish state and civilians of Lebanon inevitably caught up in the fighting. The Biden administration should encourage that instinct. It has expressed frustration at the Netanyahu government for insufficient flexibility about a cease-fire deal for Gaza. But U.S. officials also acknowledged in recent days that Hamas leader Yahya Sinwar’s intransigence has been as much, or more, to blame.

Here is the hope, at any rate: Israel’s strikes on Hezbollah represent enough of a win for Mr. Netanyahu, and enough of a defeat for Iran and its proxies, Hamas included, that all parties might conclude they have nothing more to gain from fighting, but plenty to lose.