U.S. and Israeli intelligence agencies are working to determine whether Israel’s expected ground offensive against Hamas in the Gaza Strip could prompt Hezbollah to launch a large-scale military campaign against Israel from Lebanon, American and Israeli officials said.
American officials said they believe the deployment of two carrier strike groups, each of which consists of an aircraft carrier, its planes and several escort warships, has — for now — appeared to deter Hezbollah from attacking Israel in a major way. Israel has also reinforced its northern border after the attack by Hamas on Oct. 7, in which 1,400 people were killed.
Israeli and American officials currently assess that Hezbollah’s leader, Hassan Nasrallah, does not want an all-out war with Israel, for fear of the damage it would do to his group and Lebanon. U.S. officials said that assessment could change as more intelligence is gathered and events unfold.
In addition, Prime Minister Benjamin Netanyahu of Israel has vetoed proposals from his government of a pre-emptive strike against Hezbollah, according to American officials and others briefed on the discussions.
Keeping the war confined to Gaza is a key American and Israeli priority. A significant campaign by Hezbollah, an Iranian-backed Lebanese Shiite group, would force Israel to fight simultaneously on two fronts, a difficult feat. It could also possibly draw the United States into the conflict, potentially by launching airstrikes against Hezbollah targets.
Immediately after the Hamas attack, U.S. and Israeli intelligence agencies concluded that Mr. Nasrallah was surprised by the scale and intensity of the assault. It was part of a growing body of evidence that neither Hezbollah nor Iran helped plan such a major attack by Hamas, said U.S. and allied officials, who spoke on condition of anonymity to divulge sensitive details about the crisis.
Spy agencies had long assessed that Mr. Nasrallah, whose group fought a 33-day war with Israel in 2006, did not want an all-out conflict with Israel, despite Hezbollah’s persistent anti-Israel rhetoric.
Some Iranians have disputed those assessments, arguing Mr. Nasrallah helped plan the Hamas attack. And Western intelligence is imperfect. U.S. and Israeli officials, for example, did not believe Hamas wanted to launch such a major operation against Israel before the attack on Oct. 7.
U.S. officials are increasingly concerned that Mr. Nasrallah will come under pressure from the group’s hard-line members to engage in the kind of full-scale war he has publicly called for but, U.S. and Israeli officials said, he has privately sought to avoid.
What intelligence agencies are trying to determine is whether Mr. Nasrallah is now more likely to take actions he had previously avoided, what those actions would be and whether the threat of direct American involvement on Israel’s side will be enough to keep him on the sidelines, and if so, for how long.
Before the attack on Oct. 7, according to Israeli officials, Mr. Nasrallah had considered Israel at a particularly weak point in its history. But Israel’s reaction to the attack, including the intense barrage of airstrikes on Gaza that have killed 2,808 people, and the preparations for a ground offensive, could have shifted Hezbollah’s strategy, a senior Israeli defense official said.
This is one reason current American intelligence agencies believe that Mr. Nasrallah wants to keep his organization out of a major war, U.S. officials said.
Still, Israel’s northern border has been tense.
Clashes along the border with Lebanon — the most serious since the 2006 war — and Israeli airstrikes inside Syria have stoked fears of a wider conflict in the region.
Israel has evacuated the northern border and reinforced it with military units to deter any potential attack, but clashes broke out on Sunday and Monday. Hezbollah fired at an Israeli tank and other positions on Monday, while Israel responded with artillery fire.
Though serious, Hezbollah’s attacks have mostly been fairly contained.
Hezbollah appears to have calculated that the kinds of attacks it has carried out so far are enough to show solidarity with Hamas, but not enough to provoke a large-scale response by Israel, according to an Israeli defense official.
But Israeli officials cautioned that the northern front remains a vital concern. Mr. Nasrallah could be pressured to step up attacks. If an errant strike kills a large number of people, Israel could respond with far greater force, according to Israeli officials.
Hezbollah poses a markedly more serious threat than Hamas because of its vast arsenal of precision-guided missiles and thousands of experienced fighters.
“There is a risk of an escalation of this conflict, the opening of a second front in the north and, of course, of Iran’s involvement,” Jake Sullivan, President Biden’s national security adviser, said on CBS News on Sunday.
“He wants to send a very clear message of deterrence to any state or any actor that would seek to exploit this situation,” Mr. Sullivan added, referring to Mr. Biden.
In 2006, Hezbollah fighters ambushed an Israeli border patrol, killing three soldiers and capturing two more, leading to weeks of fighting that left more than 1,000 Lebanese, mostly civilians, and roughly 165 Israelis, mostly soldiers, dead.
The war was viewed on both sides as having mixed results, entrenching Hezbollah in Lebanon as a powerful military and political force but also causing enormous damage in Lebanon.
Since then, Hezbollah has expanded its arsenal of rockets and missiles, many provided by Iran or procured with support from Tehran. While the border has remained volatile, Israel and Hezbollah have largely managed simmering tensions in order to prevent another major escalation.
For the past eight years, for example, the Israeli military has attempted to avoid killing Hezbollah fighters, even as it has struck other targets in Syria and Lebanon.
Israeli officials believe their strategy for managing the conflict with Hezbollah has largely been successful.
U.S. and Israeli intelligence agencies buttressed that assessment with their analysis that Mr. Nasrallah was wary of provoking another all-out war, which he believed could deal significant damage to his organization and weaken his power. To reduce the chances of that happening, Mr. Nasrallah has kept cross-border attacks rare and relatively small, Israeli officials said.
In turn, Israeli officials said they believed that Israel’s muted responses to Hezbollah’s provocations reduced pressure on Mr. Nasrallah to escalate the conflict further, shortening each cycle of violence and allowing calm to be restored.
American officials said Mr. Nasrallah’s next moves would likely depend on how the Israeli ground war in Gaza unfolds. U.S. and Israeli officials fear Mr. Nasrallah may no longer be able to resist the pressure to open a northern front as the number of Palestinian casualties rise during the ground invasion.
“If Hamas looks like it will be destroyed, Hezbollah will have incredible pressure to get directly involved and open a northern front,” said Mick Mulroy, a former senior Pentagon official and C.I.A. officer. “The more civilians killed, the more outrage will come from people in the region. This will put more pressure on Hezbollah to join the fight or lose credibility.”