As Iran and the United States on Saturday meet again for diplomatic talks over Tehran’s nuclear activities, few will be watching the outcome as closely as Israel.

Israel has long relied on President Trump to take a hard line against Iran, which has called for the destruction of the Jewish state. And during his first term in office, he did, ordering the killing of a top Iranian security official, devastating Tehran’s economy with American sanctions and abandoning an international accord limiting Iran’s nuclear program.

But now, as Mr. Trump resists being pulled into a new war in the Middle East, he is trying a more measured approach.

While the specter of military action remains — “If we have to do something very harsh, we’ll do it,” Mr. Trump said this past week — he has pushed ahead with negotiations and is said to have asked Israel to hold off from attacking Iran’s nuclear sites.

Mr. Trump’s chief envoy to the talks, Steve Witkoff, has also sent mixed signals over what the United States wants. Over the past week, he has moved from suggesting Iran could have a limited nuclear program to saying Iran needed to fully dismantle it.

And on Friday, Secretary of State Marco Rubio said that any deal must keep Iran from ever possessing a nuclear weapon. “It has to be something that not just prevents Iran from having a nuclear weapon now, but in the future as well,” he told reporters on a trip to Paris.

All of that has put Israel on edge over what Saturday’s talks will produce — and to what extent it may be newly vulnerable. The discussions will be held in Rome but mediated by Omani diplomats, who hosted the first round of talks last week.

“The question is, what exactly is the U.S. going to insist?” the Israeli diplomat Michael Herzog, who stepped down in January as Israel’s ambassador to Washington, said this past week. “What model is it going for as a diplomatic outcome?”

A question of red lines.

Iran, too, has tempered its expectations for the renewed diplomatic efforts over the next two months, under a deadline set by Mr. Trump.

“This must be pursued carefully,” Iran’s supreme leader, Ayatollah Ali Khamenei, said in a statement posted Tuesday on social media. “The red lines are clear. They’re clear for the other side, and they’re clear for us too.”

He said he was “neither overly optimistic nor overly pessimistic” about the process, but described the first round of indirect talks as good.

Iran maintains that its nuclear program is legal and meant only for civilian uses, like energy and medical isotopes. It refuses to stop enriching uranium, the material needed to make a nuclear bomb.

For more than a decade, world leaders have sought to limit Iran’s uranium enrichment to levels far below what is necessary to be weaponized. That requires independent verification from outside inspectors. On Wednesday, the head of the United Nations atomic agency, Rafael Grossi, was in Tehran to urge its leaders to cooperate.

Scientists believe Iran is closer than ever to being able to produce six or more nuclear weapons in months, or perhaps a year.

After meeting last week with Iran’s foreign minister, Abbas Araghchi, Mr. Witkoff suggested Iran might be able to produce low levels of uranium under a verification system that would also extend to Tehran’s missile program. But within a day, Mr. Witkoff pivoted to insist that “Iran must stop and eliminate its nuclear enrichment and weaponization program” for any deal to be struck.

Experts said Mr. Witkoff’s shift reflects the uncertainty in the Trump administration — and most likely of the president himself — over whether it thinks it can strike a reasonable deal.

The Trump administration might be open to allowing low enrichment as long as Iran offers an unprecedented concession of its own, like opening its nuclear sites to U.S. inspectors or welcoming American investors, said Ellie Geranmayeh, a senior Middle East policy expert at the European Council on Foreign Relations.

“My sense is that the Iranians and the Americans are still at a stage where they are defining one another’s red lines,” said Ms. Geranmayeh, who has been analyzing Iran’s nuclear diplomacy for more than a decade. “And it would be really critical, soon, for the Americans to come out with a final position.”

She also said that Iran had never accepted full nuclear dismantlement or zero enrichment, and “if this is where the Americans go, we are heading toward a military confrontation.”

That may be what Israel and its allies in Congress want. “It is in their interest to push the conversation between the Iranians and the U.S. toward a dead end,” Ms. Geranmayeh said.

A ticking clock.

Israel believes it has a narrow window of opportunity to strike Iran’s nuclear sites at a time when Tehran is weakened militarily and economically.

Almost all of the attack plans that Mr. Trump has asked Israel to shelve would have required the United States to take a central role to ensure its success and defend Israel from retaliation. A military operation could take weeks, if not months, to conclude, with no guaranteed outcome.

Most experts believe Tehran will try to extend the talks past Mr. Trump’s two-month deadline — in part to find common ground for full technical negotiations, but also potentially to prepare its own military for conflict.

If that happens, Israel is likely to tell Mr. Trump “that time is running out,” said Michael Makovsky, a senior Pentagon policy adviser during the George W. Bush administration.

With Iran’s nuclear program advancing, and the memory of hesitating when Hamas militants attacked on Oct. 7, 2023, in the deadliest single day in Israeli history, “they’ve learned they just can’t wait any more,” he said.