The nuclear deal with Iran caused fury in Israel and consternation around the region at the likely increase in influence and resources of a newly enriched Iran.
Most telling was the loudest expression of support. "I am happy that the Islamic Republic of Iran has achieved a great victory by reaching an agreement," President Bashar al-Assad of Syria said in a message to his Iranian opposite number, Hassan Rouhani.
"In the name of the Syrian people, I congratulate you and the people of Iran on this historic achievement."
Israel and the Sunni Arab world have set aside old grievances to stand together against the West’s engagement with Iran.
The more strident denunciations came from Israel, which regards Iran as a direct threat. Benjamin Netanyahu, the prime minister, said the country would not be bound by what he called a "stunning historic mistake".
"Israel is not bound by this deal with Iran," he said in a televised address hours after the conclusion of the accord. "Iran continues to seek our destruction and we will defend ourselves."
Benjamin Netanyahu during an earlier, tense press conference (Reuters)
Mr Netanyahu, who had condemned the deal even before it had been announced, said its terms failed to achieve the goal of denying Iran the capacity to build a nuclear bomb while, by lifting sanctions, enabled its theocratic rulers to increase their support for groups Israel considers terrorists.
"The bottom line of this very bad deal is exactly as Iran's President Rouhani said today - the international community is removing the sanctions and Iran is keeping its nuclear programme," he said.
Saudi Arabia regards Shia Iran as a competitor for leadership in the Muslim world, and sees its hand behind many of the region’s conflicts. The two are supporting opposite sides in wars in Syria and Yemen, while Saudi Arabia and its Gulf allies fear Iran’s influence among their own Shia populations.
Saudi officials have previously voiced fears that increasing rapprochement between Washington and Tehran could eventually lead to Iran supplanting Saudi Arabia as America's main ally in the Persian Gulf.
Some Western supporters of the deal hope that having been brought “into the circle of nations” Iran will become what they call a “constructive player” in Middle East regional negotiations.
That means, in Syria’s case, agreeing to a deal whereby Mr Assad is forced out in favour of a transitional government representing all non-jihadist factions in the civil war.
Estimates of the cost to Iran of propping up Mr Assad with cash, military advisers and Shia fighters hired from across the region range from $6-35 billion annually.
"Iran must show that it is ready to help us on Syria to end this conflict," Francois Hollande of France said.
However, there is nothing in the deal that would force Iran to change its stance on Syria or any of the other conflicts, such as in Yemen, where it is backing the Houthi rebels against the recognised, Saudi-backed government.
Last month Hassan Rouhani, the Iranian president, was quoted as saying: "The Iranian nation and government will remain at the side of the Syrian nation and government until the end of the road."
Torbjorn Soltvedt, an analyst with the Maplecroft risk advisory group, said Iran had an ambition to establish itself as the dominant power in the Gulf and beyond.
“Iran will remain in conflict with Saudi Arabia in Syria and Yemen in particular,” he said.