SHOMERA, Israel—Israeli infantry and artillery units are digging in along the country’s northern border with Lebanon, reinforcing a stretch of frontier that could become a dangerous second front if Iran-backed Hezbollah militants launch broader attacks.
Deadly, but so far sporadic, clashes with Hezbollah have already erupted in the area, as the bulk of the Israeli military is massing in southern Israel, poised to launch a major ground offensive in Gaza against Hamas militants who have killed more than 1,300 Israelis since last weekend.
If Hezbollah—which is designated a terror group by the U.S. and is closely linked to Iran—enters the conflict in a concerted way, it would stretch Israel’s army and strain the country’s air defenses, given Hezbollah’s arsenal of missiles capable of striking Tel Aviv.
Hezbollah has privately warned Israel that it is prepared to open a second front if Israeli forces launch a ground assault in Gaza, according to Arab officials and diplomats. But whether Hezbollah follows through with the threat is a central topic of debate for Israeli leaders.
The U.S. has in turn warned Hezbollah and Iran that it will defend Israel if the Lebanese militant group launches a major attack. Israel’s defense minister vowed to send Lebanon back to “the Stone Age” if Hezbollah attacked.
“We’re on the sword’s edge,” said one former Israeli defense official. “Brinkmanship is a dangerous way of life.”
There have been fights on this border every day since Hamas carried out its weekend attack down south, and anxiety is mounting. Thousands of residents have fled for safer ground. Those that have stayed behind fear a new war is coming.
On Wednesday night, those concerns reached a fever pitch when Israeli soldiers along the border got word—which turned out to be false—that Hezbollah had launched a major attack, putting soldiers and civilians on edge and highlighting the depth of the tensions and risks of miscalculation by either side.
In a pine forest near the border late Wednesday, two Israeli soldiers took up fighting positions outside a cramped concrete bomb shelter as their radio crackled with urgent warnings.
“All signs indicate that we are under a coordinated attack,” came the voice on the radio. Air-raid sirens and automatic gunfire echoed through olive and pomegranate orchards and Israeli drones buzzed overhead. Israel’s Home Front Command ordered Israelis living near the Lebanon border to find immediate shelter.
“Brace yourselves,” one soldier said to the half dozen others gathered at the bomb shelter door as a handful of anxious Israeli citizens huddled inside.
Israeli officials said Thursday that the military had mistakenly detected more than 10 hang gliders entering the country from Lebanon, triggering the alarms.
Hezbollah entered the fray on Sunday by launching several mortars into Israel. Israeli strikes into Lebanon killed three Hezbollah fighters on Monday. Hezbollah responded with an attack that killed the deputy commander of the Israeli army brigade in the region and two soldiers. On Tuesday, Hezbollah fired antitank missiles at Israeli military positions on the border. On Wednesday, Israel and Hezbollah again traded fire at the border, about 10 miles from the Mediterranean Sea.
A two-front war with rockets coming from the north and south would put massive strains on Israel’s much-vaunted Iron Dome air-defense systems, according to Yoel Guzansky, a senior researcher at Tel Aviv University’s Institute for National Security Studies who previously worked on Iran issues on Israel’s National Security Council.
“Hezbollah is a different story [to Hamas],” he said. “Buildings in Tel Aviv will fall. Hundreds of people will die in Tel Aviv. They have many more capabilities than Hamas.”
Israeli leaders are trying to deter Hezbollah from joining the fight. “Do not make a mistake,” Defense Minister Yoav Gallant said while visiting troops in northern Israel on Tuesday. “If, God forbid, an escalation or confrontation erupts here, we will return Lebanon to the Stone Age.”
Residents across northern Israel are bracing for the worst. Most of the women and children from the small Israeli border town of Shomera have left while men guard the town with automatic weapons.
“Something bigger is expected to come,” said Gilad Samipur, one 35-year-old resident of Shomera, a small community separated from Lebanon by a small valley. Shomera overlooks the spot where Hezbollah launched a cross-border attack on an Israeli military patrol in 2006, sparking a deadly 34-day war.
In that conflict, Hezbollah launched some 4,000 rockets in attacks that killed 120 members of Israel’s military and at least 40 civilians. Israeli planes carried out nearly 12,000 missions that killed more than more than 1,100 Lebanese civilians and fighters.
Since then, Iran has helped Hezbollah to develop an even more powerful arsenal with an estimated 150,000 missiles, including hundreds believed to have the ability to effectively target Israel’s major cities.
Israeli soldiers spread out along the Lebanese border vowed to deliver a punishing blow to Hezbollah if the militant force decides to launch a major attack on Israel. Troops here are angry about losing friends and family in last weekend’s Hamas attacks.
Assaf Maimon, an Army reservist stationed along the Lebanese border, said Israel should act ruthlessly to eliminate the threats it faces on any front.
“As far as I’m concerned, a ‘phase change’ is needed: to initiate a ‘transfer’, to cast them to sea, to massacre them,” Maimon said Wednesday. “I know that this sounds bad—and we are not murderers. But there isn’t much else to do.”
Eliran Giladi, a 39-year-old resident of Shomera, said the anger wasn’t about exacting revenge on innocent civilians, but rather on militants.
“We are not like them,” he said as he took a break from guarding an entrance to the border town. “We are not animals like them. When you say revenge: I definitely want to kill whoever killed our innocent people, our babies, our women. Even ISIS didn’t do what they have done.”
Summer Said in Dubai contributed to this article.
Write to Dion Nissenbaum at dion.nissenbaum@wsj.com