Pointedly delivering his first foreign speech in Riyadh on Sunday, Trump praised the six-state Gulf Cooperation Council, specifically Saudi Arabia, for efforts to cut off funding to extremists while lambasting Iran as the "spearhead of terrorism".
While the unpredictable Trump completed the visit to the region without committing a major gaffe, his approach "swept away the entire Obama legacy with the wave of a hand," said Thierry Coville, an Iran expert at the French Institute for International and Strategic Affairs.
"That the United States wants to restore a certain balance in its ties with Saudi Arabia, OK, but to put Iran and the IS group on the same level is false, illogical and counter-productive," Coville told AFP.
Coville said that removing human rights concerns routinely levelled at Arab states from the centre of US foreign policy was "understandable".
But he added: "Telling (Egypt's President Abdel Fattah al-Sisi) that he is doing a 'fantastic' job and saying that Saudi Arabia is a 'magnificent kingdom', while forgetting that in Iran people are making the effort to get out and vote, is very dangerous."
Hasni Abidi, head of the Geneva-based Study and Research Centre for the Arab and Mediterranean World (CERNAM), also spoke of a "break in US policy".
Obama, whose secretary of state John Kerry helped broker the 2015 Iran nuclear deal, "banked on Iran's moderate forces, (while) Trump has chosen the Sunni camp, starting with Saudi Arabia, a country with unequalled financial means," Abidi told AFP.
Washington and Riyadh agreed deals totalling some $380 billion (340 billion euros).
"But the price of regaining the confidence of Saudi Arabia and the countries of the Gulf Cooperation Council is to portray Iran as a threat," Abidi added.
He noted that Washington still depends on Iran for cooperation in Iraq against IS and in Afghanistan against the Taliban, and Trump has "shown caution so far".
Coville went further, calling Trump's speech a "smokescreen" aimed at "making people forget that he is in the process of validating the nuclear accord".
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He also noted that Iran Air, in Tehran's first deal with a US aviation group since the 1979 Islamic revolution, finalised in December a contract worth $16.6 billion to purchase 80 Boeing planes.
Paris-based political scientist Azadeh Kian said authorities in Tehran had not taken Trump's Riyadh speech seriously, pointing to a tweet by Foreign Minister Mohammad Javad Zarif saying that Washington was "milking" Saudi Arabia.
But she said it was "worrying, especially after an election... that showed that there is a real dynamic towards democratisation and opening in Iranian society."
- 'More hope than experience' -
Meanwhile Trump's call for Israelis and Palestinians to "make a deal" has drawn praised mixed with a healthy dose of scepticism.
Speaking in Jerusalem, the US leader offered no specifics but said he was "personally committed" to helping to resolve the decades-old conflict.
"Making peace, however, will not be easy," Trump said. "But with determination, compromise and the belief that peace is possible, Israelis and Palestinians can make a deal."
The Israeli newspaper Haaretz said Trump had "succeeded in bringing peace, which in recent years had become a dirty word, back to the centre of Israeli public and political discourse."
But it added: "Trump has the will and motivation, but for now he has no plan."
Former Middle East peace negotiator Aaron David Miller, writing in Politico last week, argued that "strange as it may seem, Trump actually has a Middle East strategy, or at least a reasonably coherent approach."
The idea is to forge a "US-Israeli-Sunni Arab entente... held together by several objectives they share in varying degrees: destroy ISIS, roll back Iranian influence, and deliver some kind of Israeli-Palestinian peace," Miller wrote.
It is a "transactional deal" in which the Arab states would help Washington revive the peace process in exchange for a much tougher US policy against Iran, he said.
However, he warned: "But Washington's approach seems driven more by hope than experience. Trump is likely to discover that the region... is more complicated than he thought."