Clashes in the last year between Israel and the Hezbollah militant group in southern Lebanon have largely been confined to a tit-for-tat border conflict — until now. In recent days, Israel assassinated several top Hezbollah leaders in the Lebanese capital of Beirut and bombarded sites across the country, claiming more lives in a single day than since at least 2006.

And Hezbollah has sent a barrage of rockets, less lethal but hitting ever deeper into Israeli territory, forcing thousands to flee and stoking an Israeli sense of vulnerability.

The escalation is fueling fears that a limited conflict will soon expand beyond many borders with calamitous consequences.

What are the chances of all-out war?

By many measures — the intensity of airstrikes, the number of dead, the rhetoric — there is already a significant war raging between Israel and the Lebanese militant and political group, which the U.S. classifies as a terrorist organization.

Neither side has declared it formally, but a steady escalation of cross-border attacks has raised fears of of an all-out war between two battle-scarred adversaries.

Some U.S. and other diplomats continue to insist that neither Israel nor Hezbollah — or, importantly, Hezbollah’s backer Iran — wants the conflict to widen, given the potential for it to spiral catastrophically out of control. Other observers, however, predict that momentum has already passed a point of no return and full-scale war is inevitable.

Why are they fighting?

Hezbollah and Israel have been bitter enemies for decades. Each is dedicated to the other’s destruction.

But Hezbollah now says it has stepped up its attacks on Israel because of that country’s war in the Gaza Strip, where health officials say more than 41,000 Palestinians have been killed. The Hamas militant group that ruled in Gaza triggered the war when it invaded southern Israel on Oct. 7, killed about 1,200 people and seized more than 200 hostages.

On Oct. 8, Hezbollah declared its solidarity with Hamas and started sending rockets and drones over northern Israel. It has since vowed to continue attacks until there is a Gaza ceasefire.

For its part, Israel says the proximity of tens of thousands of well-armed Hezbollah fighters poised on its northern border is an untenable threat. With more sophisticated weapons than it has ever had, Hezbollah has been able to fire rockets ever deeper into Israeli territory.

Haven’t they gone to war before?

Yes, and there have been other conflicts along the Israeli-Lebanese divide predating Hezbollah, underscoring the volatile nature of the border and mutual historical hatreds.

The most bitter entanglement started in 1982 when Israel invaded southern Lebanon and its troops marched all the way to Beirut. Israeli forces were battling armed Palestinian militants who had been shelling Israel from outposts in Lebanon.

Hezbollah did not yet exist but formed after 1982 in response to the Israeli invasion.

Although Israel soon withdrew from the outskirts of Beirut, it continued to occupy southern Lebanon for nearly two decades, until it finally pulled out in 2000.

In 2006, Israel and Hezbollah fought a war that lasted just over a month and killed about 1,300 Lebanese (Hezbollah militants and civilians) and 165 Israelis, while causing major destruction in southern Lebanon and in some parts of northern Israel.

What are the chances this war would spread?

The biggest fear in Washington and some Arab capitals is that a broadening conflict will pull in other nations.

An eventual confrontation between Iran — which backs Hezbollah — and the U.S. — which backs Israel — is a scenario that both have wanted to avoid at all costs.

Iran wants nothing more than to undermine Israel but has always sought to do so through proxies, including Hezbollah and Hamas, with Hezbollah being the most important of all of them.

At some point, however, Tehran may fear that a raging war in Lebanon would cut off its valuable land bridge to the Mediterranean Sea, a mighty incentive for keeping the conflict at bay.

The last thing the U.S. wants, in an election year especially, is to be dragged into yet another war — one that would bring potential nuclear dangers as well as the possibility of a far wider conflict.

There’s the rub.

Hezbollah says it will stop its attacks on Israel only if there is a permanent ceasefire between Israel and Hamas — something the top diplomats from the U.S., Egypt and Qatar have spent many unsuccessful months trying to do.

The diplomats claim they are close, but increasingly it seems that neither Israeli Prime Minister Benjamin Netanyahu nor Hamas leader Yahyah Sinwar is truly interested in ending the fighting on anything but his own absolute terms.

The Biden administration has refused to put more pressure on Israel or deny it weapons, while it is unclear how much pressure Qatar and other Arab states are putting on Hamas — or whether either side would even listen.

In that same vein, Israel under Netanyahu has apparently paid little heed to U.S. entreaties not to escalate hostilities with Hezbollah. President Biden, in his address Tuesday to the U.N. General Assembly, again urged “a diplomatic solution” that he said was “still possible.” “An all-out war benefits no one,” Biden said.

But no prospects for revived diplomacy appeared imminent.

What would a war look like?

As horrific as the Gaza war has been, an Israeli-Hezbollah war could be worse.

As Iran’s most valued proxy and with the resources that brings, Hezbollah has more fighters and far superior weaponry than Hamas. Fighting could be much deadlier, especially on the Israeli side, because Hezbollah would be better equipped to attack deep inside Israel.

Israel, even if somewhat taxed after months of military operations in Gaza, has a commanding advantage in air power. In recent days it has delivered stunning blows to Hezbollah’s leadership and communications — by assassinating several commanders in Beirut and remotely blowing up pagers and other handheld devices used by Hezbollah. The attacks have suggested Israel may have penetrated Hezbollah security.

In theory, the number of civilian casualties in Lebanon could be fewer than in Gaza, because the Lebanese, unlike Gazans, are not hemmed into a small geographical area with little possibility for escape or refuge. Still, in the 2006 conflict, Israel laid waste to numerous Lebanese villages.

Ultimately, to wage a full-scale war, Israel would launch a ground invasion into Lebanon as it did in the 1980s, to historically disastrous results. Whether this would lead to a longer-term Israeli occupation, or the establishment of an internationally patrolled neutral zone, or some other arrangement, is as unknown as the other outcomes of the ongoing conflict.