Both Israel and Hezbollah quickly claimed victories of sorts: Israel for its predawn pre-emptive strikes against what the military said were thousands of Hezbollah’s rocket launcher barrels in southern Lebanon; and Hezbollah for its subsequent firing of barrages of rockets and drones at northern Israel, which the Israeli military initially said had caused little damage.

By breakfast time, the two sides were employing the language of containment.

Hezbollah announced that it had completed the “first stage” of its attack to avenge the assassination of the senior commander, Fuad Shukr, and appeared to be calling it a day, at least for now. Israel’s defense minister, Yoav Gallant, said he had spoken with U.S. Secretary of Defense Lloyd J. Austin III and they had “discussed the importance of avoiding regional escalation,” according to a statement from Mr. Gallant’s office.

Still, the Middle East remained on edge, the days ahead uncertain.

“There can be stages,” cautioned Ehud Yaari, an Israel-based fellow of the Washington Institute for Near East Policy, a research group. “You can have escalation that is gradual.”

Later Sunday morning, the Israeli military said it was continuing to strike Hezbollah launchers in southern Lebanon. Hezbollah is estimated to possess tens of thousands of rockets and a smaller number of more sophisticated, precise missiles.

And Iran, Hezbollah’s patron, still has an open account with Israel, blaming it for the assassination of Ismail Haniyeh, the political leader of its ally Hamas, while he was in Tehran, just hours after the killing of Mr. Shukr. Israel officially took responsibility for Mr. Shukr’s death but not for Mr. Haniyeh’s.

Based on intelligence, Israel took the decision to pre-empt Hezbollah’s attack on Sunday “but not to go beyond,” Mr. Yaari said. The targets that Israel struck were all less than 30 miles inside Lebanon, he said. Israel said they were focused on thwarting Hezbollah’s immediate attack plans, not its wider assets or infrastructure.

Hezbollah, for its part, appears to be “signaling that it is done for now,” Mr. Yaari said. “At the same time, they are saying this was the first stage of retaliation, leaving open the option to do more, if they get a green light from the Iranians,” he added.

The events on Sunday have raised the stakes for negotiators gathering in Cairo to try to advance a cease-fire and hostage release deal for the ongoing war in Gaza. The United States is leading the push, along with Qatari and Egyptian mediators, for a deal that would end the 10-month conflict between Israel and Hamas, in the hope that such an agreement could help calm tensions in the region.

Hezbollah and Israel had already engaged for months in tit-for-tat cross-border clashes. Hezbollah began firing in solidarity with Hamas after last October’s Hamas-led assault on southern Israel prompted Israel to go to war in Gaza.

The exchanges between Israel and Hezbollah have grown in intensity in recent weeks, in what many analysts have described as a war of attrition.

With both sides attempting to project success, some analysts said Hezbollah’s attack appeared to have been a bust. Israel said that there had been minimal damage from the attacks, and it did not immediately report any casualties. Three people were reported killed in Israeli strikes in southern Lebanon on Sunday, according to Lebanon’s Health Ministry.

What happens next is contingent on several factors, analysts said, including the comments expected later on Sunday by Hassan Nasrallah, the Hezbollah leader. The armed group said Mr. Nasrallah would deny Israel’s claim to have thwarted a major attack, suggesting that he would seek to claim a victory that could in effect close this chapter of hostilities.

“They don’t want an escalation. They don’t want a war,” Mohanad Hage Ali, a Beirut-based fellow at the Carnegie Middle East Center, said of Hezbollah. He said the attack provided Hezbollah with an off-ramp to avoid a wider escalation, even if it did not go as intended.

“They want to say that we’ve registered a response, and now move on from this phase of anticipation of a wider escalation,” he said.

Still, he said, “this would be a failure, militarily speaking,” for Hezbollah, the best armed of Iran’s array of regional proxy forces.

“If Hezbollah did not cause casualties on the Israeli side, and deep into their territory, this is not really a response.” Mr. Hage Ali said.

Some Hezbollah supporters in Lebanon agreed.

“If this is the response for Shukr’s death, I think that they will lose a lot of support from the public,” said Mohammed Awada, 52, a taxi driver who lives in Beirut’s southern suburbs, an area of the Lebanese capital where Hezbollah holds sway. “They did not even get close to their promise,” he added, referring to Hezbollah’s earlier pledge to strike Tel Aviv if Beirut was hit.

Sabah Suleiman, a resident of Beirut’s southern suburbs who was near the site of Mr. Shukr’s assassination last month, said she was pleased that Hezbollah had retaliated, though she feared a further response from Israel.

“I can’t say I’m not afraid,” she said in a text message, adding that she couldn’t flee the area because she couldn’t afford to live elsewhere.

“We have no option except to stay and wait, but Sayed Nasrallah gave us hope” with Sunday’s attack, she said, using an honorific for the Hezbollah leader.

Kassem Kassir, a Lebanese analyst with ties to Hezbollah, said the group’s attack was “an initial response, but the battle is not over.”

“No one can determine the course of the confrontation,” he said, adding: “Sayed Nasrallah will speak today — we are all waiting.”

5 a.m. (11 p.m. Eastern on Saturday): The Israeli military says that its fighter jets have begun bombarding targets in Lebanon belonging to Hezbollah. The military “identified the Hezbollah terrorist organization preparing to fire missiles and rockets toward Israeli territory,” it says.

In a video statement, the Israeli military’s chief spokesman, Rear Adm. Daniel Hagari, says that Israel had begun “a self-defense act to remove these threats,” describing Hezbollah’s preparations as “extensive.” Ben-Gurion International Airport near Tel Aviv is closed to flights.

5:32 a.m.: Air-raid sirens blare across northern Israel, warning of an incoming rocket barrage. The Israeli news media circulates footage showing Israeli air defenses intercepting rockets fired from Lebanon and describes the barrage as longer than has been typical in the months of intensified launches by Hezbollah.

6:09 a.m.: Hezbollah confirms that it launched an attack as part of an “initial response” to the Israeli assassination of Fuad Shukr, one of the group’s most senior commanders, last month. The group says it targeted Israeli military bases and aerial defense batteries, and fired drones toward a significant but unspecified military target.

6:20 a.m.: Israel’s defense minister, Yoav Gallant, announces that a state of emergency is in place, limiting public gatherings. Mr. Gallant’s office says he spoke by phone with Defense Secretary Lloyd J. Austin III to update him on Israeli actions “to thwart an imminent threat against the State of Israel.”

6:30 a.m.: Admiral Hagari, at a news briefing, says Israel is “removing threats to the Israeli home front.” He signals that attacks aiming deep into Israel may have been neutralized, saying that Ben-Gurion airport will soon reopen for flights.

6:55 a.m. Hezbollah releases a second statement saying it completed the first phase of its attack. It says the assault included launching both attack drones and 320 rockets at 11 military installations in northern Israel. The rockets were aimed at Israeli military sites in an attempt to facilitate the drones’ passage toward targets deeper inside Israel, according to Hezbollah.

8:12 a.m.: The Israeli military says that about 100 Israeli fighter jets were involved in the Israeli operation, which attacked and destroyed thousands of Hezbollah rocket launch barrels in southern Lebanon, most of which had been aimed toward northern and central Israel. Nadav Shoshani, a military spokesman, tells reporters Hezbollah fired hundreds of drones and rockets but caused little damage.

8:26 a.m.: Prime Minister Benjamin Netanyahu of Israel releases a statement with his remarks to a cabinet meeting a short time earlier. He reiterates his pledge to create the conditions for tens of thousands of Israelis displaced from their homes by Hezbollah attacks near the border to go back home. “We are determined to do everything to protect our country, to return the residents of the north safely to their homes and continue to uphold a simple rule: Whoever harms us, we will harm them,” he said, according to his office.

8:58 a.m.: Hezbollah releases a new statement saying its military operations were “finished for the day,” and denies Israel’s claim that it had thwarted a major attack. It says its leader, Hassan Nasrallah, will speak about the day’s events Sunday evening.

11:08 a.m.: Admiral Hagari says that Israel’s strikes against Hezbollah are continuing and that “several areas in southern Lebanon” were hit over the previous hour. The pace and intensity of the Israeli strikes appears to be slowing.

12:42 p.m.: The Israeli military lifts the restrictions on public gatherings that it had imposed hours earlier.

Hezbollah later said it had fired hundreds of rockets at Israel, though Israel said it had largely thwarted the barrage.

The two sides have repeatedly traded strikes since the Gaza war began in October, killing civilians and combatants in Lebanon and Israel. The hostilities have also forced civilians on both sides of the border to leave their homes.

Here’s a look at Hezbollah and its capabilities.

What is Hezbollah?

Hezbollah was founded in the 1980s, after Israel, responding to attacks, invaded and occupied southern Lebanon, intending to root out the Palestine Liberation Organization, which was then based in the country.

Hezbollah’s guerrilla fighters quickly grew effective at bedeviling the far-better-equipped Israeli forces. By 2000, Israel had withdrawn from Lebanon, making Hezbollah a hero to many Lebanese. It fought Israel again in 2006, launching a military operation into its southern neighbor that led to a fierce counterattack. In that war, Israel rained bombs on southern Lebanon and Beirut, the capital; the fighting killed more than 1,000 Lebanese.

Yet, the Israeli military never managed to overwhelm Hezbollah in 34 days of war, allowing the group and its leader, Hassan Nasrallah, to emerge as stars in the Arab world.

Hezbollah soon allied with Iran, and they became close partners.

Hezbollah, considered a terrorist group by the United States and other countries, has evolved from a fighting force into a dominant political one, accruing significant influence in Lebanon’s government.

How strong is Hezbollah?

Through propaganda videos and calibrated strikes, Hezbollah has repeatedly displayed signs of a bulked-up arsenal that analysts say is capable of inflicting heavy damage on Israel. Its forces are also battle-tested after years of fighting against rebels in Syria, where Hezbollah sent thousands of fighters to help prop up the government of President Bashar al-Assad.

Estimates vary on just how many missiles Hezbollah has and how sophisticated its systems are. The Central Intelligence Agency’s World Factbook says the group may have more than 150,000 missiles and rockets of various types and ranges. It also estimates that the group has up to 45,000 fighters, though Mr. Nasrallah has claimed to have 100,000.

But analysts and Israeli officials say Hezbollah’s arsenal is considerably more dangerous than Hamas’s because of its precision-guided missiles, which could target critical Israeli infrastructure and military assets.

Hezbollah has also displayed exploding drones that can elude Israel’s Iron Dome, the system designed to protect the country from incoming rockets and missiles. The group also appears to have anti-tank missiles that fly too fast and too low for the Iron Dome to intercept.

Euan Ward contributed reporting.

Here are some of the key developments in recent months.

Jan. 2: A Hamas leader’s killing in Beirut

Hamas accused Israel of killing Saleh al-Arouri, a senior leader, along with two commanders from its military wing in an explosion in a suburb of Beirut, Lebanon’s capital. Previously, Beirut had been far from the cross-border violence between Israel and Hezbollah, the powerful Lebanese militia that, like Hamas, is aligned with Iran. Mr. al-Arouri was the first high-level Hamas official to be killed outside the West Bank and Gaza in recent years. Israeli officials declined to comment, but Lebanese and U.S. officials attributed the attack to Israel.

Jan. 6: Hezbollah’s retaliation against Israel

In response to Mr. al-Arouri’s assassination, Hezbollah fired a volley of rockets at a small military base in northern Israel. Though Hezbollah said it caused casualties, the Israeli military reported no injuries and responded with its own strikes in Lebanon. Analysts viewed Hezbollah’s response as a symbolic act rather than a significant escalation, with the group firing about 40 rockets toward Mount Meron, an area housing a military radar station.

April 1: Airstrikes in Damascus

Israel carried out airstrikes that hit part of the Iranian Embassy complex in Damascus, Syria, killing three senior Iranian commanders and four officers involved in Iran’s covert operations. The attack, one of the deadliest in the yearslong shadow war between Israel and Iran, increased regional tensions, which were already strained over the war in Gaza and hostilities involving Iran-backed groups. Israeli officials, speaking anonymously, confirmed the strike but denied that the targeted building had diplomatic status.

April 14: A barrage of missiles and drones against Israel

Iran retaliated for the Damascus strikes by launching more than 300 drones and missiles against Israel, its first open attack on Israel from Iranian soil. The strikes, aimed at military targets, caused minor damage and injured a young girl. Israel intercepted most of the projectiles and others were shot down by U.S. and Jordanian defenses. The calibrated attack, telegraphed well in advance, demonstrated Iran’s effort to avoid mass casualties or direct war, analysts said.

July 13: An airstrike in a designated humanitarian zone of Gaza

Israel tried to kill Muhammad Deif, a top Hamas military commander in Gaza, in an airstrike that the territory’s health ministry said killed 90 people and injured 300 others. The strike hit a strip of coastal land known as Mawasi, which Israel had designated as a humanitarian zone, and where thousands of displaced Palestinians were living. Mr. Deif, believed to be a mastermind behind the Oct. 7 attack on Israel, had long been a high-priority target, and after weeks of uncertainty about his condition, Israeli authorities said in August that he had been killed. Hamas has not explicitly confirmed or denied Israel’s claim.

July 27: Rocket strikes in the Israeli-controlled Golan Heights

A rocket from Lebanon struck a soccer field in the Druse town of Majdal Shams in the Israeli-controlled Golan Heights, killing 12 teenagers and children, according to the Israeli military. It was the deadliest single attack from across Israel’s northern border in months of hostilities. Israel accused Hezbollah, but the group denied responsibility.

July 30: A second strike in Beirut

Israel targeted Fuad Shukr, a senior Hezbollah leader and close adviser to the organization’s leader, Hassan Nasrallah, in a deadly strike in Beirut. Israeli officials described the attack as a response to the Golan Heights rocket strike, but the assault quickly raised concerns in the region about Israel’s willingness to strike deep within Lebanese territory.

July 31: A top Hamas official is killed in Tehran

Hours after the strike in Beirut, Ismail Haniyeh, one of the most senior Hamas leaders and a key figure in the stalled cease-fire talks, was assassinated in Iran, where he had gone for the inauguration of that country’s new president. Iran and Hamas said Israel had carried out the killing, and they vowed to retaliate. Mr. Haniyeh, who had led Hamas’s political office and helped manage negotiations for a cease-fire, was killed by an explosive device covertly smuggled into the guesthouse in Tehran where he was staying, according to seven Middle Eastern officials, including two Iranians, as well as an American official.

Aug. 25: Israel says it thwarted a major attack

The Israeli military said it had launched airstrikes against Hezbollah forces that had been preparing an “extensive” attack against Israel, destroying what it said were thousands of rocket launch barrels. Hezbollah later said it had retaliated for Mr. Shukr’s killing by launching hundreds of rockets at Israeli military positions — believed to be one of its largest barrages in months — though Israel said there had been “very little damage.”

The strikes came after cross-border strikes between Israeli forces and Hezbollah militants in Lebanon had escalated sharply. A week earlier, an Israeli airstrike hit a factory in a small town in southern Lebanon, killing at least 10 civilians, according to Lebanese officials. It was one of the largest death tolls in a single strike in Lebanon since the war in Gaza began.

Yan Zhuang contributed reporting.

Two aircraft carriers — the Theodore Roosevelt and the Abraham Lincoln — and their accompanying warships and attack planes are now in or near the Gulf of Oman. Mr. Austin also made public his order to send the guided-missile submarine Georgia to the region, an unusual move as the Pentagon seldom talks about the movements of its submarine fleet. The Georgia can fire cruise missiles and carry teams of Navy SEAL commandos.

The orders came in response to threats from Iran and its proxies in Gaza, Lebanon and Yemen to attack Israel to avenge the assassination of a top Hamas leader, Ismail Haniyeh, in Tehran on July 31.

While the United States has said these moves are to help defend Israel and avert a wider regional war, a senior U.S. official said on Saturday night that the American military was better positioned to address a threat from Iran, and that the Israeli Defense Forces would shoulder the bulk of any defense from attacks carried out by Hezbollah across the border in Lebanon.