"I know how things work in Israeli intelligence," said Uri Bar-Joseph, a professor at Haifa University in Israel who has studied and written widely about the Jewish state's spy operations. "I have some friends I talk with. They're upset. They are sincerely frustrated and angry."
Meeting Russia's foreign minister and ambassador to Washington in the Oval Office last week, Trump shared intelligence about an Islamic State threat involving laptops carried on airplanes, according to a senior US official who wasn't authorised to talk about the sensitive material and spoke on condition of anonymity.
But some of the people who've spent years safeguarding that relationship say there will be consequences.
Trump made "two very serious mistakes", former CIA director John Brennan said today at a financial industry event in Las Vegas.
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"We shared a lot of sensitive intelligence about terrorism operations that were planned against the Russians," he said. "But we shared it through intelligence channels, and you also make sure that the language of what you are sharing is not going in any way compromise your collections systems. Mr Trump didn't do that."
Bar-Joseph, the writer, said: "I won't say they won't share secrets anymore, but when it comes to the most sensitive information, there will be a second thought." Of Trump, he added, "If you can't count on the president, who can you count on?"
Both nations gain much from the exchange of information.
Washington, in turn, provides Israel with financial and military assistance, and intelligence that US agencies collect on threats far beyond Israel's immediate borders.