The White House appeared reliable as long as the United States was imposing economic sanctions on Iran, and threatening the country with retaliation if it resorted to violence in response.

But the White House has not been quite so dependable more recently, Israelis say.

The Trump administration’s failure to hit back at Iran after repeated strikes on oil tankers and Saudi oil fields that were widely, if not undeniably, attributed to Tehran has undermined the credibility of American military threats, Israeli analysts said.

Mr. Trump’s openness to talks with Iran has reinforced the idea that he is averse to a new conflict in the region. And his pullout of troops from Kurdish territory has only reinforced the broader perception among Israelis that he wants to withdraw from the Middle East, even at the expense of American influence.

“There’s a growing sense that Trump is backing away from his commitments to allies,” said Emily B. Landau, an arms-control expert at the Institute for National Security Studies in Tel Aviv. “I’m not sure Israel’s in the same category as Saudi Arabia and the Kurds. At least I’m hoping that we’re not in the same category. But expectations were forged through Trump’s rhetoric and his behavior, and some of his policy decisions. And the question is, to what degree will he follow through with it, if Israel really needs the United States?”

That American dependability is even being questioned by Israelis could embolden Iran at a particularly dangerous time, Israeli analysts said.

“We are already in a highly volatile period, with Iran attacking U.S. allies,” like Saudi Arabia, said Ofer Zalzberg, an Israeli analyst for International Crisis Group. “The Israelis are bracing against an Iranian attack. The defense establishment believes Iran will strike within two months. The Israeli reaction would be very different from the Saudi nonreaction, and Iran knows that. But it’s very dangerous to encourage Iran to feel safer and to give Iran more courage in its decisions.”