JERUSALEM — Prime Minister Benjamin Netanyahu of Israel and his former challenger, Benny Gantz, agreed Monday night to establish a unity government, a deal that finally breaks a yearlong political impasse and keeps Mr. Netanyahu in office as he faces trial on corruption charges.

After three inconclusive elections in the past year, the creation of the new government forestalls what had appeared to be an inevitable fourth election and offers a deeply divided Israel a chance for national healing as it battles the coronavirus pandemic.

For Mr. Netanyahu, the agreement buys him time to try to resolve two contentious issues central to his legacy: to sidestep his prosecution or at least prevent it from driving him from power, and to extend Israeli sovereignty over occupied Palestinian territory.

The deal, announced by both men’s political parties at 7:15 p.m., adds to Mr. Netanyahu’s tenure as Israel’s longest-serving leader and, coming after his conservative coalition failed to win a majority, cements his reputation as a canny political survivor who can never be counted out.

For Mr. Gantz, a former army chief and relative political novice, however, the agreement may have the opposite effect. The move was a stunning turnabout after his repeated campaign vows that he would never serve with a prime minister under criminal indictment, and a disappointment to many of his supporters who saw it as a capitulation to a leader they had wanted to oust.

Under the deal, which the two leaders cast as an emergency government to fight the coronavirus, Mr. Gantz will be named deputy prime minister and is to get a turn as prime minister halfway through their three-year term, in October 2021, switching roles with Mr. Netanyahu.

But given Mr. Netanyahu’s political cunning, Israeli analysts were skeptical that he would hand over power when the time came — or even that the new government would last that long.

Mr. Gantz won at least a measure of insurance: Their rotation agreement would require 75 of the 120 members of Parliament to vote to overturn it, and if the government is disbanded early he would become prime minister.

But Mr. Netanyahu, whose trial on bribery, fraud and breach of trust charges is now set to begin May 24, will have an effective veto over appointments of the next attorney general and state prosecutor.

Mr. Netanyahu had also sought an assurance — which he did not achieve — that if the Supreme Court rules that a criminal defendant cannot serve as prime minister, the new government would pass legislation overriding such a decision.

But Mr. Netanyahu did win a clause saying that such a decision by the court in the next six months would immediately lead to new elections. Mr. Netanyahu could then try again for a 61-seat majority that would enact the override he would need to retain power.

Anshel Pfeffer, a biographer of Mr. Netanyahu, said he expected that, rather than try to torpedo the deal when Mr. Gantz’s turn comes as prime minister Mr. Netanyahu had reason to protect it because he is still assured of one of the top two jobs in Israel.

“He’s not only won this, he’s bought Gantz’s services as a bodyguard,” Mr. Pfeffer said. “Gantz is bound to protect Bibi, because it’s the only way he becomes prime minister,” he said, using Mr. Netanyahu’s nickname.

“And for the next three years, Bibi can go on trial, they can say the most terrible things about him in court, and he can lose, but until he loses his last appeal, he’s protected,” Mr. Pfeffer said.

Despite the sense of urgency to form a joint government to fight the virus sweeping through the country, the final sticking points in the negotiations were political.

According to representatives of Mr. Gantz’s centrist Blue and White party, they included differing approaches to President Trump’s proposal to resolve the Israeli-Palestinian conflict: Mr. Netanyahu has pledged to swiftly and unilaterally annex large swaths of the occupied West Bank while Mr. Gantz has said he would support annexation only with international consensus, which appears highly unlikely.

The final agreement delays consideration of annexation until July 1 at the earliest. It declares that annexation must be done in a way that safeguards Israel’s interests, “including the needs for preserving regional stability, protecting existing peace agreements and aspiring for future ones.” But it leaves those determinations up to the government and says that Mr. Gantz is only entitled to “consultation” with Mr. Netanyahu on annexation, not a veto.

The two-month delay gives opponents of annexation, who say it would reignite the conflict and deal a death blow to already dim hopes of a two-state solution, a brief window to build international and domestic support to try to block such a move.

“A government coalition based on a commitment to annex more occupied Palestinian territory is a threat to a rules-based world order in general, but to peace, security and stability in the Middle East in particular,” Saeb Erekat, the veteran Palestinian negotiator, said in a statement. “Annexation means the end of any possibility for a negotiated solution.”

The deal between Mr. Gantz and Mr. Netanyahu first envisions an emergency government lasting for six months, when only legislation related to the battle against the coronavirus can be taken up by Parliament — except for bills concerning the Trump plan, the two sides said. A “coronavirus cabinet” will be formed, jointly led by Mr. Netanyahu and Mr. Gantz, to coordinate efforts.

The Trump administration plan, announced in January, gives Israel most of what it has sought over decades of conflict while offering the Palestinians a state with limited sovereignty. Under the plan, Israel would control a unified Jerusalem as its capital and would not be required to uproot any West Bank settlements.

For now, it offers a vital lifeline to Mr. Netanyahu, who can fight his corruption trial from the prime minister’s office.

By taking aggressive steps to combat the coronavirus, Mr. Netanyahu had already gone a long way to reasserting his leadership and varnishing his international profile.

“I promised the state of Israel a national emergency government that would work to save the lives and livelihoods of Israeli citizens,” he wrote on Twitter. “I will continue to do everything for you, citizens of Israel.”

Mr. Gantz’s assent to a joint government led to the immediate breakup of his party, dismantling the most formidable adversary Mr. Netanyahu faced during his last 11 years in office.

Many of Mr. Gantz’s former supporters have accused him of betraying them by joining Mr. Netanyahu’s right-wing and religious coalition. Some analysts said the decisions may have ended Mr. Gantz’s nascent political career.

Mr. Gantz has argued that given the public health crisis, it was the only responsible choice.

“We prevented a fourth election,” he said in a Twitter post Monday night. “We will preserve democracy. We will fight the coronavirus and take care of all of Israel’s citizens.”

The coronavirus is known to have infected more than 13,600 Israelis, and has claimed at least 173 lives so far. The country has been under lockdown, with potentially devastating economic consequences, and is only beginning to allow some businesses to resume operation under strict conditions.

The deal announced Monday divides the numerous ministries of Israel’s government in half. Mr. Gantz and his allies get defense, justice, strategic affairs, economy, labor, communications, agriculture, culture, tourism and diaspora affairs, among others. The spoils for Mr. Netanyahu’s cohort include health, finance, interior, internal security, transportation and housing. The foreign ministry will shift hands from Mr. Gantz’s bloc to Mr. Netanyahu’s after 18 months.

The primary goals of Mr. Gantz’s campaigns were to unseat Mr. Netanyahu and to uphold the rule of law after years of vitriolic attacks by Mr. Netanyahu and his supporters on the police and the judiciary.

Despite Mr. Gantz’s vows to protect the judiciary, his former partner in Blue and White, Yair Lapid, noted scornfully that Mr. Netanyahu had come away with control over a key committee that oversees the appointment of Supreme Court judges, who must retire at 70.

“So the compromise in the judge-appointing committee is that Bibi chose all of the committee’s representatives,” Mr. Lapid wrote on Twitter. Mr. Gantz, he added, had “agreed to allow the criminal defendant to appoint all of the judges who will discuss his case. There is no limit to the disgrace.”

After the last election, on March 2, a diverse, Gantz-led array of anti-Netanyahu forces, ranging from Arab and Islamic parties to Jewish ultranationalists, gained a slim majority of 61 parliamentary seats. Cooperating to form even a minority government proved impossible, but Mr. Gantz and his allies did at first agree to try to deny Mr. Netanyahu yet another term, either by enacting term limits or by passing a law to bar an indicted lawmaker from forming a government.

Those efforts are now dead, and Mr. Netanyahu still towers above every rival as he has for as long as many Israelis can remember.

“Even giving Gantz the benefit of the doubt that it’s better to be a part of the government, Bibi has broken up Blue and White, the only party that’s come close to challenging him in the past 10 years,” said Tal Shalev, political writer for the Walla News website. “Gantz doesn’t have any achievements that come close to equaling the huge score that Netanyahu has just gotten.”

Adam Rasgon contributed reporting.