Following the announcement of an agreement in Vienna last week, the White House dispatched Carter overseas to try to reassure Iran's arch-enemies that the agreement makes them more safe.
So far, Carter doesn't seem to have succeeded.
Following a seemingly warm meeting with Israeli Defense Minister Moshe Ya'alon on Monday, which included a tour along Israel's northern border with Lebanon to showcase the threat from Hezbollah, his meeting with Israeli Prime Minister Benjamin Netanyahu Tuesday did little to convince the Israeli leader on the components of the deal.
"The prime minister made it quite clear he disagrees with us with respect to the nuclear deal," Carter told American and coalition troops fighting ISIS at a base in nearby Jordan later in the day. "But friends can disagree."
The two men shook hands for the cameras but skipped the microphone set up for a joint appearance and spent the rest of their meeting behind closed doors.
Going into the trip, Pentagon officials said Carter was not traveling to Israel to change Netanyahu's mind about the deal but rather to reinforce the U.S. commitment to Israel's security, to address bilateral security challenges and to discuss how the U.S. could bolster cooperation in addressing regional threats.
A senior defense official told reporters traveling with Carter said that while Netanyahu spoke passionately about his opposition to the deal, he did not come across as being angry and that Carter had expected to receive his opposition.
The official added the discussion did not get into the politics of the deal, and that Netanyahu made clear that he would prefer the deal not to happen.
Defense officials said the trip was planned before the Iran deal was finalized and would also focus on maintaining Israel's military edge, along with countering Iran's destabilizing activities in the region.
During the visit, Carter laid a wreath at Yad Vashem, the Holocaust memorial in Jerusalem.
It was Carter's first trip to Israel since becoming Defense Secretary and the first cabinet-level visit since Netanyahu's reelection earlier this year.
In Jordan, while visiting a base used by the United States and its coalition partners to strike ISIS targets inside Syria, Carter met with a group of pilots and even spoke with one sitting in the cockpit of an F-16.
Carter travels to Saudi Arabia Wednesday to meet with King Salman and other senior officials to discuss regional threats such as ISIS, along with cyber cooperation, maritime security and missile defense. Pentagon officials said Carter would not be announcing any weapons package for the Saudis during his visit.
The Sunni-led kingdom has made its concerns known about the prospect of a deal with Shiite Iran, so its criticism of the new agreement is likely to come up in private meetings.