WASHINGTON - US President Barack Obama is sending his defense chief next week to the Middle East, where he faces the unenviable job of reassuring allies such as Israel that a nuclear deal with Iran will not undermine America's commitment to their security.
It could be a very hard sell.
So far, the White House has only disclosed one stop on the trip - to Israel, where Prime Minister Benjamin Netanyahu condemned Tuesday's deal as "a stunning, historic mistake".
US defense officials told Reuters that Defense Secretary Ash Carter would travel elsewhere within the region but declined to offer details.
The agreement between Iran and six major world powers could transform the Middle East, curbing sensitive Iranian nuclear activities in exchange for sanctions relief - and, in the process, upending assumptions about Tehran's isolation.
Iran is the predominant Shi'ite Muslim power, hostile both to Israel and to Washington's Sunni Muslim-ruled Arab friends, particularly Saudi Arabia. Allies of Riyadh and Tehran have fought decades of sectarian proxy wars in Syria, Lebanon, Iraq and Yemen.
Even US military officials concede that unchaining Iran's economy from crippling sanctions will likely translate into more money for Iran's military and its surrogates abroad.
Offering a hint of his message to allies, Carter said in a statement about the Iran deal that the United States stood ready to "check Iranian malign influence".
"We remain prepared and postured to bolster the security of our friends and allies in the region, including Israel," he said.
Philip Gordon, who until March was the top White House National Security Council official on the Middle East, said there needed to be an explanation that the nuclear deal was not what allies such as Israel feared: "Which is blanket reconciliation with Iran."
If US allies in the region say they need more defense and intelligence cooperation, "then that needs to be part of the discussion," he said.
Obama made a flurry of phone calls with regional allies on Tuesday, including with the crown prince of Abu Dhabi and King Salman of Saudi Arabia. The White House said Obama stressed a commitment to work with Gulf partners to counter Iran's"destabilizing activities" and help them bolster their defense capabilities.
The prospect of an increasingly assertive Iran has seen Gulf states build up their militaries, including with US weaponry. Obama hosted leaders of the Sunni Arab states in the United States in May, vowing to back them against any external attack.