Defense Secretary Jim Mattis testifies before the Senate appropriations defense subcommittee hearing on funding for the Department of Defense on May 9, 2018. (Yuri Gripas/Reuters)

Defense Secretary Jim Mattis held out hope that renewed efforts to curtail Iran’s nuclear program in consultation with European nations could still work, a day after President Trump announced his withdrawal from the Iran nuclear pact, upsetting some of the Pentagon’s most important allies abroad. 

Mattis, who advocated staying in the agreement despite his hawkish views on Iran, said on Wednesday that the United States now had an opportunity to address the deal’s shortcomings and “make it more compelling” in consultation with allies. He said Secretary of State Mike Pompeo and Treasury Secretary Steven Mnuchin had been working on that, and he suggested that they would continue to do so. 

Though Mattis and other top Pentagon leaders had called the agreement with Iran flawed, they advocated staying in the 2015 deal and revising its most problematic aspects. They said publicly that Iran was complying; and late last month, Mattis described the agreement’s verification regime as “pretty robust,” even as Trump repeatedly called it a terrible deal. 

The retired Marine Corps general, who was replaced as commander of Central Command during the Obama administration in part because of his hard-line views on Iran, said the Trump administration had walked away from the deal “because we found it was inadequate for the long-term effort” to counter Tehran’s malicious behavior.

He said the United States would continue to work with its allies to curb what he described as Iran’s malign behavior, including its nuclear enrichment program, cyberattacks, terrorist activities, ballistic missile efforts and threats to international commerce in the Red Sea and Persian Gulf. 

The U.S. military is now faced with the need to dust off a strategy to deal with the possibility that Iran could return to its pursuit of nuclear weapons in the wake of the agreement’s collapse. Iran has threatened to restart its nuclear enrichment program, but the country could still try to abide by the terms of the agreement if it can secure continued sanctions relief and investment from European nations under a revised deal that doesn’t include the United States. 

Army Gen. Joseph Votel, who oversees American operations in the Middle East as commander of U.S. Central Command, suggested that the military would have to consider the possibility of using force if the deal were to fall apart and Iran were to return to its pursuit of nuclear weapons — a military operation that repeated analyses have determined to be complicated and far from foolproof.

The agreement “addresses one of the principal threats that we deal with from Iran, so if the [agreement] goes away, then we will have to have another way to deal with their nuclear weapons program,” Votel said in March without going into detail. He said there would be concern in the region about how the United States intended to address the threat if not through the deal. 

Like Mattis, Votel had pushed for the United States to stay in the deal. So, too, had Marine Corps Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff.

Michael Connell, director of the Iranian Studies Program at the Center for Naval Analyses, said any more provocative behavior stemming from Iran in the region as a result of the deal’s falling apart could prevent the U.S. military from moving resources away from the Middle East toward Asia and Europe, as the Pentagon had hoped to do to account for competition with China and Russia.

“That’s going to be tougher if Iran is misbehaving,” Connell said.

The Pentagon declined to say whether the United States would be adjusting its forces in the region to cope with any fallout, including the possibility of heightened tension between Israel and Iran.

Lt. Cmdr. Rebecca Rebarich, a spokeswoman for the Pentagon, declined to say whether the military was stepping up protections for American forces in the region, citing operational security reasons. “We don’t want to give that information away to our enemies, who watch very closely,” she said.