Millions of Venezuelans head to the polls on Sunday for a presidential vote that represents a pivotal moment for the socialist movement that has governed the oil-rich nation for 25 years.

Over the course of a generation, Chavismo, as the movement is called, has shattered the nation’s democracy, presided over an extraordinary economic contraction unlike any seen outside of war, and become the source of one of the largest migrant crises in the world.

The election, held on the birthday of the movement’s founder, Hugo Chávez, pits Mr. Chávez’s successor, President Nicolás Maduro, against the previously little-known Edmundo González, a former diplomat.

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The surveillance drone appeared high above the Ukrainian air base without warning in early July. Minutes after it relayed targeting data back to a Russian base, a barrage of ballistic missiles struck the airfield, Ukrainian officials said, recounting the episode.

“That first hit was so powerful that even our windows were trembling,” said Valeria Minenko, 21, who lives near the air base in Myrhorod, central Ukraine, one of many targeted in relentless attacks by Russia in recent months.

“Now they’re hitting the air base with the rockets all the time,” Ms. Minenko said.

Russia has been saturating the skies over Ukraine with surveillance drones, exploiting gaps in air-defense systems, to launch increasingly sophisticated attacks on Ukrainian positions. Its dominance in the air along parts of the front has allowed it to bombard Ukrainian positions with hundreds of powerful guided bombs every day, helping its ground forces to make slow and costly gains.

Ukraine’s strategy was to counter Russia in the air war with the aid of long-coveted F-16 fighter jets from the West that it says it will deploy this summer.

But the assaults on Ukrainian air bases underscore Russia’s determination to limit the impact of the planes even before they enter the fight. They also highlight the challenges Ukraine faces as it prepares to deploy the sophisticated aircraft for the first time.

Ukraine is hoping the F-16s, which come with powerful electronic warfare systems and an array of other weapons, can be used in coordination with other Western weapons like Patriot air-defense systems to expand the area deemed too dangerous for Russian pilots to fly. They also hope the jets will add another layer of protection for Ukrainian cities and critical infrastructure from relentless missile and drone attacks.

But a shortage of trained pilots and a limited number of jets will constrain the immediate impact, experts say.

“Russia has had so much time to fortify its defenses, especially along the frontline areas,” said Hunter Stoll, a defense analyst at RAND, a research organization. “The F-16s and their pilots will face stiff resistance from Russian air defenses, both on the ground and in the sky.”

Ukraine says it is “in the process” of moving the first F-16s into the country, about two and a half years after it first pleaded for the aircraft. It has been a year since the Biden administration finally reversed policy and allowed Western allies to transfer American fighter jets to Ukraine.

“Today, we can already say clearly, we have entered the club of countries that have F-16s,” Yuri Ihnat, a representative for the Ukrainian Air Force, said in an interview. “This is a turning point for our nation.”

The arrival of the planes — the exact number has not been publicly revealed — comes at a moment of deep uncertainty in the war. Russian forces are engaged in furious assaults all along the 600-mile front, the Ukrainian energy grid is crippled by years of unrelenting bombardment and a presidential election in the United States could reshape future military assistance.

In addition to the Russian attacks on the Ukrainian airfields, Ukraine will also be constrained by the small number of trained pilots, according to Ukrainian and U.S. military officials. About 20 airmen in the various U.S., Dutch and Danish training pipelines are expected to be ready this year, according to U.S. officials.

Air commanders say they typically allot at least two pilots per aircraft — for crew rest, training and other matters. So that would allow Ukraine to fly only about 10 F-16s, at most, on combat missions this year.

Another major limiting factor, these officials say, is the number of trained maintenance and support personnel on the ground to keep the F-16s flying.

“It’s not just the pilots you have to have,” Gen. Charles Q. Brown Jr., the chairman of the Joint Chiefs of Staff and a longtime F-16 pilot, said last month. “Maintenance is also a key part of that, and training the maintainers.”

Gen. Serhii Holubtsov, chief of aviation of Ukraine’s air force, said Ukrainians “do not wear rose-colored glasses” and understand that the F-16 is “not a panacea.”

The strategy, he told Donbas Realiy, a branch of Radio Liberty, can be thought of in three phases — “crawl, walk, run” — and it will take time.

“We haven’t learned to crawl yet,” he said.

Before the jets can start to play a role in shaping the battlefield, Ukraine needs to be sure they can be protected. While Russia has been attacking Ukrainian airfields since the first hours of the war, the early July attack on Myrhorod was different, Ukrainian officials said.

“The enemy came up with a new tactic,” Mr. Ihnat said.

Specifically, he said, the Russians are improving missiles and reconnaissance drones, “making it so that we cannot influence them with electronic warfare.”

They are also preprogramming surveillance drones to fly deep into Ukraine without emitting telltale electronic signatures, making them harder to detect.

Gen. Oleksandr Syrsky, Ukraine’s top military commander, said this past week that Ukraine urgently needed to find new methods of destroying enemy drones.

Mr. Ihnat said that the Ukrainian air force had effectively adopted deception tactics — like building model planes to act as decoys, camouflaging aircraft and moving them — to protect its depleted fleet of Soviet-era aircraft, and would do the same for the F-16s.

“If someone wants to laugh at this, let them,” he said. “Thanks to the models, the enemy has already lost dozens or even hundreds of their missiles.”

Ukraine is also employing 1970s-vintage Yakovlev Yak-52 training planes to hunt Russian surveillance drones, he said.

The propeller-driven aircraft have been hunting Russian surveillance drones across southern Ukraine, with both Ukrainian and Russian forces posting videos of the aerial clashes.

General Holubtsov said he expected attacks on the airfields to increase. For that reason, he said, Ukraine will not keep all the promised F-16s in the country.

“There are a certain number of aircraft that will be stored at secure air bases, outside of Ukraine, so that they are not targeted here,” he said. “And this will be our reserve in case of need for replacement of faulty planes during routine maintenance.”

President Vladimir V. Putin of Russia has said that the storage abroad of planes or other Ukrainian military assets could “pose a serious danger of NATO being drawn further into the conflict.”

The Biden administration’s approach to arming Ukraine has been driven in large part by concerns about potential escalation with Moscow, which is why it resisted allowing the transfer of F-16s from allies for so long.

Retired Lt. Gen. David A. Deptula, the dean of the Mitchell Institute for Aerospace Studies in Washington, said the delay “has given Russia the gift of time.”

“We gave them time to dig in and establish defenses that are now much more difficult to unravel,” he said.

American, Dutch and Danish officials have been working with Ukrainian counterparts to hammer out the details of synchronizing the arrival of the promised aircraft, equipping them with air-to-air and air-to-ground munitions, and thinking through the most efficient and effective use of the initial group of planes, the U.S. and Ukrainian officials said.

After spending some time getting used to the aircraft, General Holubtsov said, the F-16s can be used in the effort to push back the Russian attack planes that have been bombing Ukraine.

General Holubtsov said that F-16s alone would not be enough to drive back the Russian warplanes. They will work in concert with ground-based air defenses like the Patriots, coordinating efforts with a powerful Western information exchange network called Link 16.

The process will take time, he said, and there are a host of factors that could complicate the effort, including Ukraine’s shortage of air-defense systems, which it needs desperately to protect civilian as well as military assets.

But if the Russian planes can be driven further back from the front, the general said, “it can be considered a turning point and a victory — if not superiority, then parity in the air space.”

Dzvinka Pinchuk contributed reporting. Nataliia Novosolova contributed research.

Western diplomats were scrambling on Sunday to prevent a surge of fighting along the Israel-Lebanon border, officials said, after a rocket from Lebanon on Saturday killed at least 12 people in an Israeli-controlled town, most of them children. The rocket prompted Israel to retaliate early Sunday with strikes across Lebanon.

The initial Israeli response appeared to stop short of a major escalation, but there were still fears that the fallout from the rocket launch would lead to all-out war. Prime Minister Benjamin Netanyahu of Israel, facing domestic pressure to mount a fiercer response, was set to meet with senior government ministers on Sunday afternoon to discuss further steps, after flying back early from a trip to the United States.

Israel blamed Hezbollah, an Iranian-backed Lebanese group that has been attacking Israel in solidarity with Hamas, for the deadly rocket attack on Saturday on the Druse Arab town of Majdal Shams. Hezbollah has denied it was responsible.

Secretary of State Antony J. Blinken said at a news conference on Sunday in Tokyo that there was “every indication” that the rocket was fired by Hezbollah.

U.S. diplomats were working on Sunday to contain the hostilities and asked Lebanon’s government to relay a message to Hezbollah to show restraint in the face of a further Israeli response, according to Lebanon’s foreign minister, Abdallah Bou Habib.

“We are trying to restrain Hezbollah now from retaliating to whatever the Israelis do next,” Mr. Bou Habib said in a call with The New York Times. It was not immediately possible to confirm that with U.S. officials.

French officials also passed messages back and forth between Israel and Hezbollah, according to a Western official who requested anonymity because of the sensitivity of the issue. France still has some influence in Lebanon owing to its former status as a French protectorate after World War I.

The backchannel diplomacy came amid threats from both Israel and Iran. The Iranian foreign ministry warned Israel of “unforeseen consequences” of any Israeli escalation, while Israel’s education minister, Yoav Kisch, called for a strong response “even if it means entering into an all-out war.”

The Israeli military said its overnight strikes had chiefly targeted places in Lebanon that it had often hit in the past, mostly close to the border with Israel or surrounding the southern port of Tyre. It reported one strike in the Bekaa Valley, roughly 60 miles north of the Israel-Lebanon border, where it has been striking less frequently since February.

Lebanon’s state-run news agency reported extensive damage and some casualties resulting from the overnight Israeli strikes that began shortly after midnight and lasted until dawn. It was not immediately clear if the casualties were civilians or militants.

The rocket strike on Saturday, which hit children at a soccer field, was the deadliest assault on Israeli-controlled territory since Israel and Hezbollah began exchanging missile and rocket fire in October.

Some Israelis want Mr. Netanyahu to authorize a full-scale ground invasion of southern Lebanon in order to deter similar attacks. But others fear that such a move would prompt a far more devastating response from Hezbollah, whose arsenal of weapons is considered larger and most sophisticated than almost any other nonstate actor in the region.

Israeli commanders are also wary of opening up a second major war while the war in Gaza is still raging. After nine months of fighting with Hamas and Hezbollah, Israel’s munitions stockpiles have dwindled, raising questions about how intense a battle it could fight in Lebanon.

For now, Israeli officials say that they are still open to a diplomatic resolution to the conflict with Hezbollah. Israel’s Foreign Ministry spokesman, Oren Marmorstein, said in a statement on Sunday that a full-scale war could still be averted through the enforcement of a never-implemented United Nations resolution from 2006 that would create a demilitarized zone in southern Lebanon.

Still, there were strong expectations on Sunday morning that Israel might mount a bigger response. That, analysts fear, could tip the low-level hostilities between Israel and militias led by Hezbollah into more intense conflict.

Roughly 100,000 people in Lebanon and 60,000 in Israel have been displaced, with scores of schools and health centers shuttered in both countries.

More than 460 people in Lebanon have been killed, most of them militants. More than 100 were civilians, including 12 children and 21 health workers, according to the United Nations and Lebanon’s Health Ministry. The fighting has killed 22 Israeli soldiers and 24 civilians, according to the Israeli government.

But unlike in Gaza, both sides have largely avoided attacks that cause overwhelming loss of life, which would in turn prompt their opponent to respond with overwhelming force.

The scale of the bloodshed on Saturday night has provided one of the strongest tests to that calculus since October.

“Hezbollah will pay a heavy price, which it has not paid up to now,” Mr. Netanyahu’s office said in an overnight statement.

U.N. officials urged Israel and Hezbollah to “exercise maximum restraint,” warning that “it could ignite a wider conflagration that would engulf the entire region in a catastrophe beyond belief,” according to a joint statement by the U.N. special coordinator for Lebanon, Jeanine Hennis-Plasschaert, and the chief of U.N. peacekeeping forces in Lebanon, Lt. Gen. Aroldo Lázaro.

The attack on Saturday set off widespread grief in the Golan Heights, where thousands of Druse Arabs observed a day of mourning on Sunday, shutting shops and other workplaces. Thousands went by bus to Majdal Shams to attend the funerals of those slain.

“The worst thing that has happened to the Druse in my memory,” said Diab Shams, 21, a Druse electrical engineering student who was traveling by bus to the town. “I’ve never seen anything like this,” he said, with tears in his eyes.

As the bus passed a Jewish Israeli town, scores of Jewish residents could be seen gathered beside the road waving both Israeli and Druse flags in a gesture of solidarity with the Druse community.

The Golan Heights is a territory once held by Syria that was captured by Israel during the Arab-Israeli war of 1967. Israel annexed the territory in 1981, a move that was not recognized by most of the world. Decades later, President Donald J. Trump recognized Israeli sovereignty there, but most countries consider it occupied territory.

Roughly 20,000 Druse Arabs live in the Golan Heights, including in the town hit by the rocket; some still consider themselves Syrian, refusing Israeli passports, while a minority have taken Israeli citizenship. Jewish Israelis began settling the territory after 1967, and more than 20,000 now live in the area.

Gabby Sobelman contributed reporting from Rehovot, Israel; Myra Noveck from Jerusalem; and Edward Wong from Tokyo.