Mr. Hariri, until he announced his resignation on Saturday, had shown no signs of planning to do so.
Hours later, on Saturday evening, a missile fired from Yemen came close to Riyadh before being shot down. Saudi Arabia later blamed Iran and Hezbollah for the missile, suggesting that they had aided the Iran-aligned Houthi rebels in Yemen to fire it.
Before the world had a chance to absorb this news, the ambitious and aggressive Saudi Arabian crown prince, Mohammed bin Salman, ordered the arrest of hundreds of Saudis, including members of the royal family, in what was either a crackdown on corruption, as Saudi officials put it, or a purge, as outside analysts have suggested.
It then emerged that the week before, Jared Kushner, President Trump’s son-in-law and adviser, who has been sent on missions both to Israel and Saudi Arabia, had visited Riyadh on a previously undisclosed trip and met until the early morning hours with the crown prince. The White House has not announced what they discussed but officials privately said that they were meeting about the administration’s efforts to forge an Israeli-Palestinian peace deal.
No one expects Saudi Arabia, which is mired in a military conflict in Yemen, to start another war itself. On Friday, Hezbollah’s leader, Hassan Nasrallah, said that Saudi Arabia had asked Israel to attack Lebanon, after essentially kidnapping Mr. Hariri.
“I’m not talking here about analysis, but information,” he said. “The Saudis asked Israel to attack Lebanon.”
He provided no evidence of his claim, but Western and regional analysts have also said that, given all the confusing and unexpected events and unpredictable players, they could not entirely rule out such a scenario.
Israeli officials, however, have been publicly predicting another war with Hezbollah, an increasingly well armed antagonist, while also vowing to do all they can to postpone it.
“There are now those in the region who would like Israel to go to war with Hezbollah and fight a Saudi war to the last Israeli,” said Ofer Zalzberg, a Jerusalem-based analyst for International Crisis Group. “There is no interest in that here.”
Prime Minister Benjamin Netanyahu has long considered Iran to be Israel’s foremost enemy, a potential nuclear threat as well as a strategic adversary seeking to convert postwar Syria into a staging ground for attacks against Israel or into a corridor to transfer missiles and other weapons to Hezbollah in Lebanon.
So Saudi Arabia’s stepped-up efforts to oppose Iranian influence in Lebanon drew measured applause in Jerusalem. But many Israelis fear that the aggressive actions by the Saudi crown prince could drag Israel into a war that it does not want.
Daniel Shapiro, a former United States ambassador to Israel, said that Israel and Saudi Arabia were pursuing similar goals at sharply different speeds and levels of proficiency.
“I’m not sure they’re aligned tactically,” he said in an interview. Prince Mohammed, he added, “seems very impatient to actually spark the confrontation.”
There are no signs of war preparations in Israel. The country is not mobilizing troops on its northern border or calling up reservists, and Mr. Netanyahu has given no indication that he sees a conflict as imminent.
Moreover, Israel’s war planners predict that the next war with Hezbollah may be catastrophic, particularly if it lasts more than a few days. Hezbollah now has more than 120,000 rockets and missiles, Israel estimates, enough to overwhelm Israeli missile defenses.
Many of them are long-range and accurate enough to bring down Tel Aviv high-rises, sink offshore gas platforms, knock out Ben-Gurion Airport or level landmark buildings across Israel.
Nor is Hezbollah necessarily hankering for battle with Israel, according to analysts who study the militant group closely. It is still fighting in Syria, where it has been backing the government of President Bashar al-Assad, and it is being drained by medical costs for wounded fighters and survivor benefits for the families of those killed, said Giora Eiland, a retired Israeli major general and former head of the country’s National Security Council.
“Hezbollah as an organization is in a very deep economic crisis today,” Mr. Eiland said. “But at the same time, the weaker they are, the more dependent they are on Iranian assistance — so they might have to comply with Iran’s instructions.”
But there have long been fears that now that the Syrian war — in which Hezbollah played a decisive role, gaining new influence, power and weapons — is almost over, Hezbollah’s enemies might seek to cut it down to size.
Mr. Nasrallah, the Hezbollah leader, implied Friday that its fight in Syria was nearly finished. If Saudi Arabia’s goal was to force Hezbollah to leave Syria, he said: “No problem. Our goal there has been achieved. It’s almost over anyway.”
World leaders have sought to tamp down tensions.
The American secretary of state Rex W. Tillerson warned Friday “against any party, within or outside Lebanon, using Lebanon as a venue for proxy conflicts or in any manner contributing to instability in that country,” a message apparently aimed at Hezbollah as well as Saudi Arabia.
President Emmanuel Macron of France left Saudi Arabia on Friday after a brief, last-minute meeting with the crown prince.
During the unexpected two-hour visit on Thursday, Mr. Macron “reiterated the importance France attaches to Lebanon’s stability, security, sovereignty and integrity,” his office said. He also discussed “the situation in Lebanon following the resignation of Prime Minister Hariri,” his office said, but provided no further details.
A group of countries and organizations interested in Lebanon’s stability met Friday with the Lebanese president, Michel Aoun, and issued a statement expressing “concern regarding the situation and prevailing uncertainty in Lebanon” and calling for Lebanon to be “shielded from tensions in the region.”
The members of the group, the International Support Group for Lebanon — including the United Nations, Britain, China, France, Germany, Italy, Russia and the United States, as well as the European Union and the Arab League — are not all on the same side of the issues at stake so the statement seemed to reflect broad international concern.
At a news conference in Dubai, the United Arab Emirates, before the meeting, Mr. Macron said he did not share Saudi Arabia’s “very harsh opinions” of Iran.
Analysts say a new war in the region is unlikely but some have warned that the increased tensions could provoke an economic crisis or even start a war accidentally. Miscalculations have started wars before, as in the 2006 war between Israel and Hezbollah.
Experts caution that Israel is often only a mistake or two from being drawn into combat.
“It’s a dangerous situation now,” said Amos Harel, the military reporter for Haaretz, the Israeli newspaper. “It only takes one provocation, another reaction, and it can get all of a sudden completely out of control. And when you add the Saudis, who evidently want to attack Iran and are looking for action, it gets even more complicated.”
Because of an editing error, an earlier version of this article misstated where President Emmanuel Macron of France met Crown Prince Mohammed bin Salman of Saudi Arabia. They met in Riyadh, not in Abu Dhabi.
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