Three Israeli officials said the attack would involve unmanned drones and missiles fired toward Israel. One of the U.S. officials said it would involve ballistic missiles, while the second said that it was unclear what kind of attack would be launched. The officials all spoke on the condition of anonymity to discuss a sensitive military matter.

Any attack would significantly raise the risk of an all-out war between Israel and Iran, including its proxies across the Middle East. For years, the two countries have fought a shadow war, with Iran seeking Israel’s destruction and Israel seeking to blunt Iran’s regional influence, destroy its nuclear program and unseat its government.

Now they are moving closer to direct confrontation, after a year of rising conflict between Israel and several Iranian allies including Hamas in Gaza, Hezbollah in Lebanon and the Houthi militia in Yemen.The rise in tensions follows Israel’s decision over the past month to escalate its attacks on Hezbollah, including by assassinating the group’s leader, Hassan Nasrallah, in an airstrike last week, and culminating in its overnight invasion of southern Lebanon, a Hezbollah stronghold.

Iran has faced rising pressure to come to Hezbollah’s aid but has been caught between the desire to protect its proxy and sustain its own prestige and influence, and the need to avoid a devastating counterattack from Israel that could wreck its nuclear program or kill senior Iranian leaders.

The three Israeli officials said that the target of the new Iranian attack would be three military air bases, as well as an intelligence headquarters north of Tel Aviv, which was evacuated on Tuesday afternoon.

Iran last fired a barrage of missiles and drones at Israel in April, after Israeli warplanes fired strikes that killed several top Iranian commanders as they visited Syria. At that time, an all-out war was avoided after both sides chose to de-escalate. Six months later, diplomats and experts say that a full-scale war is much likelier, with Israel expected to strike back hard after any new Iranian attack.

A group of Israel’s allies, led by the United States, helped intercept most of the missiles and drones in April, resulting in only limited damage to Israeli infrastructure.

Unlike the air and naval defensive effort in April that was pulled together by American commanders on the fly, the intervening months have allowed the United States and its allies to work out in detail access for U.S. warplanes to fly in the airspace of several countries, coordinate flight patterns to best knock down any Iranian drones or missiles and even review various Iranian attack scenarios, the U.S. military official said.

Still, U.S. officials said it was difficult to predict exactly how Iran might attack this time.

Helene Cooper, Farnaz Fassihi and Aaron Boxerman contributed reporting.

“The invasions served to widen the wedge between Lebanon’s political communities, and as they are linked to the country’s sects this has only worsened sectarian tension and fueled the country’s political divisions,” said Lina Khatib, an associate fellow at Chatham House, a London-based research organization.

As Israel invades for a fourth time, here is a brief look at the history of its previous invasions.

1978: Three-Month Invasion

Israel invaded southern Lebanon in March 1978, under Prime Minister Menachem Begin, partly in response to an attack by Palestinian militants based in Lebanon who landed by sea and commandeered a bus on a coastal road north of Tel Aviv, leaving 35 Israelis and an American dead. Israeli forces captured territory up to the Litani River, a few miles from Israel’s northern border.

Israel withdrew in June, handing control of the ground it had taken to a Lebanese Christian militia and a United Nations peacekeeping force that had been established under a United Nations Security Council resolution. Lebanese officials said 1,200 people died in the invasion. Israel said it had killed 350 Palestinian militants and lost 34 of its own soldiers.

The invasion did not solve Israel’s security problems on its northern border and some critics of Mr. Begin argued that Israel had squandered international goodwill by devastating a string of villages in southern Lebanon. Other commentators noted that Arab leaders, despite voluble rhetoric, provided little practical or military assistance to the Palestinians during the fighting.

1982-2000: P.L.O. Is Forced Out, and Hezbollah Is Formed

Israel launched a much bigger invasion in June 1982, attacking by land, sea and air. The immediate cause was retaliation for the shelling of northern Israel by the Palestine Liberation Organization, which was based in Lebanon, and for the shooting of Israel’s ambassador to Britain. The aim was to crush the P.L.O., which Israel blamed for the shooting, and to drive its weapons beyond the range of northern Israel while avoiding a wider war involving Syria, which had stationed its own troops in Lebanon during the civil war.

Israel uprooted the P.L.O. from the Lebanese capital, Beirut. It also occupied southern Lebanon at the start of the invasion and helped install a Lebanese government it considered more favorable. Israel’s warplanes shattered parts of Beirut in airstrikes, and the United Nations said that nearly 30,000 people, the vast majority of them Lebanese and Palestinian civilians, were killed in the first two months of fighting. Under heavy pressure, the P.L.O. withdrew thousands of its guerrillas from Beirut in August along with its leader, Yasir Arafat, and Israel’s military besieged the city.

A month later, a Lebanese Christian militia that had cooperated with Israel killed large numbers of Lebanese and Palestinian civilians at two refugee camps in southern Beirut, often using brutal methods. The massacre at the Sabra and Shatila camps, one of the most notorious in the region’s recent history, was widely condemned and led to the resignation of Israel’s defense minister at the time, Ariel Sharon. Estimates of how many people were killed vary. An Israeli commission put the figure at between 700 and 800. The Lebanese authorities put the total at around 2,000. Others have cited still higher numbers.

Again, the invasion had unintended consequences. Hezbollah was formed with Iranian help in the 1980s to fight the Israeli occupation of southern Lebanon. Hezbollah staged guerrilla attacks on Israeli soldiers, playing a large role in Israel’s decision to withdraw in 2000. Its decision to leave drew attention across the Arab world as a victory for Hezbollah. Ehud Barak, Israel’s prime minister at the time, said: “This 18-year tragedy is over.”

2006: All-Out War

Six years after it withdrew, Israel bombed Beirut and southern Lebanon and sent its armored forces rumbling over the frontier again in response to a surprise cross-border assault by Hezbollah in which eight Israeli soldiers were killed and two were captured. The operation came as Israel was also waging a military operation in Gaza to try to rescue a captured soldier, Gilad Shalit.

More than a month of fighting ensued in which Israel bombarded Lebanon, damaging infrastructure and forcing people to flee their homes. More than 1,000 people died in Lebanon, including many Hezbollah fighters, but the group claimed victory for having survived the onslaught. More than 100 Israeli troops were killed as well as dozens of Israeli civilians. The war also displaced around one million people in Lebanon and around half a million in Israel.

The invasion was launched to rout Hezbollah, but an official Israeli report issued two years later found “serious failings and flaws” by Israel’s political and military leaders. The investigation’s leader, Eliyahu Winograd, said that Israel had not won the war and had not responded effectively to Hezbollah’s cross-border rocket fire. Hezbollah claimed a “divine victory” and was lauded across the Arab world.

“As long as the military doesn’t order me to shut down, I’ll keep the place running,” he said. “It helps me maintain a sense of optimism.”

Since the Israeli military said early Tuesday that it had begun a ground operation in Lebanon, signs of war in the north are everywhere. Towns are nearly deserted, soldiers are positioned behind sandbags, military vehicles move along dirt roads and artillery is stationed in fields.

On Monday, the military had declared one area on the border a closed military zone, and on Tuesday, military police officers set up new checkpoints farther from the frontier, barring civilians from entering. Military personnel are now positioned across areas roughly four to six miles from the Lebanese border, some barricaded behind sandbags.

From their positions in empty fields, Israeli artillery batteries launch shells at Lebanese border areas, which are also under attack by Israeli warplanes and helicopters that occasionally fly overhead. Clouds of dust and smoke rise above the villages, growing thicker with each strike.

And long before last October, when Hezbollah and Israel began trading airstrikes and rocket fire across the border, bringing the war in Gaza to Lebanon’s green, fertile south.

Hezbollah, the Iran-funded Shiite Muslim militia that doubles as a major political party and social services organization, does not run Lebanon in any official sense. But under Mr. Nasrallah, it sometimes seemed as if it was the only force that mattered: a state within a state with its own military, schools, hospitals and youth programs.

Now his death has come as the latest thunderbolt to jolt Lebanon, a Mediterranean country of 5.4 million people already stuck in a dejected state of nonstop emergency.

Many say Lebanon’s current anguish began in 2019, when the economy imploded and took the country’s once-robust middle class with it. Mass anti-government protests that fall did nothing to dislodge the country’s widely loathed political class.

Others might mention 2020, the year the coronavirus further crippled the economy, and the year an enormous explosion at Beirut’s port shattered entire neighborhoods of the capital.

A good case could be made for going all the way back to the 15-year civil war that ended in 1990, which birthed the movement that became Hezbollah, and from which the country never really recovered.

All these crises and more have left Lebanon in no shape to withstand a sharply escalating conflict with Israel, like a 10-car pileup caught in the path of a tornado.

That much became obvious over the last week, when at least 118,000 Lebanese fled Israeli airstrikes in Lebanon’s south, in its agricultural Bekaa Valley and in the Hezbollah-dominated Dahiya suburbs of Beirut.

The official response was “chaos,” said Mark Daou, an independent member of Parliament, as the TV in his office played news footage of the hourslong traffic jams on the roads from the south last week.

He was not surprised the government seemed stupefied. “They have no money and they have no control over what’s happening on the ground,” he said, noting that Lebanon’s nominal military has little actual power. “They’re hostage to whatever Hezbollah decides unilaterally.”

While the government designated hundreds of public buildings as shelters for the displaced, it provided no mattresses, bedding, food or other supplies.

Information about shelters spread haphazardly through word of mouth and on WhatsApp, with little official guidance. Shelters filled quickly, leaving hundreds to sleep in public squares, a seaside promenade, a beach and under bridges when they evacuated the Dahiya suburbs after Friday’s huge airstrike on Hezbollah headquarters under the neighborhood.

As the longtime head of a group the United States considers a terrorist organization, but one that drove Israel out of southern Lebanon when the state could not, Mr. Nasrallah was a hero to some Lebanese and anathema to others. But his power was such that few can predict what the country will look like without him.

Mired in political paralysis — partly, Mr. Daou said, because Hezbollah has moved to block attempts at resolution — Lebanon has gone nearly two years without a president and has only a caretaker government.

The state provides barely any electricity, leaving everyone dependent on generators, if they can afford the fees. Many generators can power only one appliance at a time, so residents unplug refrigerators or forgo air-conditioning just to do laundry.

The financial crisis has left many people who could once afford overseas vacations, ski weekends in Lebanon’s mountains and sun-dazzled afternoons at its beach clubs nearly destitute, their savings trapped in banks that deny them access to their own money. Desperate, a few account holders have resorted to holding up bank branches to demand their own funds.

Thousands of doctors, nurses and medical technicians, as well as many young professionals, entrepreneurs, designers and artists, have left the country. Teachers routinely go unpaid; many of their students cannot afford textbooks.

“The country in many respects cannot withstand a long-term war,” said Sleiman Haroun, the president of a national association of Lebanese hospitals. Though the health care system had performed well so far, he said, he worried that there were not enough medical professionals left to cope with a sustained Israeli onslaught.

But, he added, “This is our fate. We have to face it.”

Enraged at their leaders, the Lebanese long ago stopped expecting anything from them.

Into the void left by the state have stepped private donors, individual volunteers, citizen aid groups, entrepreneurs and social-services organizations affiliated with political parties.

In wealthier pockets of the country, their efforts, along with the chic cocktail bars, nightclubs, manicured beach clubs and sophisticated restaurants, mask Lebanon’s collapse so effectively that first-time visitors are frequently taken aback by its high-functioning facade.

Residents and business owners have installed solar panels on rooftops across Lebanon to compensate for the lack of government-supplied electricity. Private donors pay for street lighting in some Beirut neighborhoods.

Over the last week, as shelters overflowed with displaced residents, a patchwork of volunteers and local aid groups rushed to fill the gap.

Just inside the gate of a private school in central Beirut last week sat Sarah Khalil, a board member who was helping to manage wave upon wave of donations — food, water, a refrigerator — arriving in the courtyard. The school’s board had opened its 50 classrooms to displaced families, and faculty, neighbors, students’ family members and other school affiliates were showing up with provisions.

“This is the only way,” she said. “We can’t rely on the government, but we surely can rely on those around us.”

At Dr. Sobhy Salah Middle School in the Bir Hassan neighborhood, the Ministry of Education unlocked the doors for displaced families. But it was the scouting organization affiliated with the Amal Movement, a major Shiite Muslim political party, that was running the shelter and gathering donated supplies.

Asked why the government had not provided more, Mohamed Jaber, a volunteer, let out a laugh.

“There’s no government to begin with,” he said. “The government will only wake up way after the war has ended.”

Families at the shelter said they had come there after hearing about it from relatives or through word of mouth. But many shelters filled quickly, including this one, leaving the latest wave of displaced arrivals with few options if they had no family or friends to take them in.

That was how several Syrian families ended up under a bridge in Beirut on Wednesday afternoon, beaten-up minibuses and gleaming SUVs honking around them. Their presence was a reminder of yet another crisis that has strained Lebanon: The country plays reluctant host to an estimated 750,000 refugees from next-door Syria, driven to Lebanon by Syria’s brutal civil war, its economic crisis and a powerful earthquake last year.

Bushra Ali, 24, stood under the bridge with her 4-year-old son, 2-year-old daughter and a black plastic bag of necessities, all they had been able to grab on Wednesday morning as they evacuated Dahiya, the Hezbollah-dominated suburb of Beirut that Israel has struck repeatedly.

Originally from Aleppo, Syria, her family came to Lebanon last year, after the earthquake in northern Syria destroyed their home. But the move had not been a success.

Her husband was laid off from a Lebanese shoe factory three months ago. Their rent was rising every month. Now bombs were falling and schools were closed, so they had decided to go back to Aleppo.

“It’s a really horrible feeling,” she said, her face crumpling as she stroked her son’s hair.

The Lebanese government appeared similarly missing-in-action after the port explosion on Aug. 4, 2020, that damaged more than half of Beirut and killed 218 people — a catastrophe that later investigations by media outlets and rights groups found was rooted in the government’s neglect, corruption and mismanagement. In the days after, while soldiers sat smoking on street corners, it was regular citizens who showed up to clean up the debris.

In the blast’s wake, a small, scruffy group of friends began distributing donations and free meals from an abandoned gas station in east Beirut. Four years later, now a full-fledged community kitchen and local aid group, Nation Station has begun delivering around 1,600 meals and sandwiches a day to shelters.

“The country, it’s already down. Like, I can’t believe we’re doing this again,” said Josephine Abou Abdo, a co-founder, who manages the crew of young staff and volunteers. “It’s back to Aug. 4 vibes.”

Four years ago, they were motivated by their own government’s inaction. Now, she said, it was Israel’s assault that was drawing Lebanese together in solidarity.

With Israel attacking them, she said, “this is the least that we can do.”

Jacob Roubai contributed reporting from Beirut.

Here is why Israel said it has sent ground forces into Lebanon:

What is Israel’s goal?

The invasion, which the Israeli military confirmed early Tuesday, is aimed at crippling Hezbollah’s forces and infrastructure in an area of southern Lebanon that the armed group controls, and where it has been known to build underground tunnels leading into Israel.

The incursion comes after two weeks of intensive Israeli airstrikes and attacks against the Iranian-backed Lebanese armed group involving exploding pagers and radios. Israeli officials said the assaults were intended in part to eliminate the group’s leadership; an airstrike on Friday killed Hezbollah’s leader, Hassan Nasrallah. But officials had also suggested the attacks could remove the need for a ground invasion.

Hezbollah has launched near-daily aerial attacks against Israeli positions since last October. The group says its attacks are in solidarity with Hamas, which attacked Israel on Oct. 7, prompting the Israeli military to go to war in Gaza.

Why is Israel targeting southern Lebanon?

Southern Lebanon is a rugged area, filled with steep valleys in which defenders can easily ambush an invading army, a factor that may have shaped Israeli military planning. The tough terrain has historically helped factions in the region defend themselves, and Hezbollah has used the geography to its advantage in past wars with Israel.

Israeli military leaders conducted months of preparatory drills for troops who could be sent to fight in the region, exercises they announced throughout the summer and into the fall.

It’s unclear how deep inside southern Lebanon Israeli forces will venture during this ground operation. On Tuesday, the Israeli military called for residents of more than two dozen towns and villages to evacuate and move north of the Awali River, which is more than 15 miles from the Israeli border at its nearest point.

What is U.N. Resolution 1701?

United Nations Resolution 1701, which ended the 2006 war, called on Israeli troops to withdraw from southern Lebanon to an area in the Golan Heights below what is known as the Blue Line, and for Hezbollah to withdraw north of the Litani River, which runs south of the Awali. The area in between the Litani and the border, where much of the fighting had taken place, was to become a buffer zone.

The resolution called for the area to be controlled by the Lebanese military and United Nations peacekeeping forces. A U.N. peacekeeping force has remained in the area, but the resolution was never fully implemented.

Hezbollah has argued that its presence in the buffer zone is justified because Israel still occupies Lebanese land. Although the U.N. deemed that Israel had fully withdrawn from Lebanon in 2000, Israel remained in an area known as Shabaa Farms, which the U.N. considers an occupied part of Syria. Lebanon and Hezbollah say the land is Lebanese.

The resolution had envisioned the demilitarization of Hezbollah, but the group has only gained political and military might. In 2008, Israel sought peace talks with Lebanon’s government but was rebuffed, in large part because Hezbollah had gained political power in an agreement with the Lebanese government.

Israel maintains that Hezbollah has built up its arsenal of missiles aimed at Israel’s northern border and has constructed more underground tunnels that would allow the group to infiltrate and attack Israel, as it did in 2006. In 2018, an Israeli military operation uncovered tunnels built by Hezbollah, prompting Israel to call for international action. U.N. forces in Lebanon confirmed the presence of tunnels.

Last month, as Israel ratcheted up its air attacks on Hezbollah, Israel’s foreign minister, Israel Katz, sent a letter to the United Nations arguing that the Israeli military aimed to thwart the armed group’s plans to infiltrate Israel and called for enforcement of Resolution 1701.

Israeli officials have long feared that Hezbollah would conduct the kind of attack that Hamas led last Oct. 7 of last year, the incursion that killed about 1,200 people and set off the war in Gaza.

Hezbollah has suffered serious blows beyond the loss of its leader in the intensified Israeli attacks that preceded the current invasion, including heavy airstrikes near and in Beirut over the weekend. But Hezbollah has continued to fire rockets on northern Israel — and occasionally into central Israel — proving its resilience.

Hezbollah is thought to have perhaps the largest arsenal of any armed group in the world, excluding governments, according to experts. A report in March by the Center for Strategic and International Studies, a research organization based in Washington, put the size of Hezbollah’s stockpile between 120,000 and 200,000 rockets and missiles, including guided ballistic missiles, short-range and midrange unguided ballistic missiles and short- and long-range unguided rockets.

Patrick Kingsley, Matthew Mpoke Bigg and Isabel Kershner contributed reporting.