President Trump fulfilled a campaign promise when he walked away from the 2015 Iran agreement. The question now is whether he can reasonably expect to negotiate a better deal with Iran, let alone with North Korea — which already has an advanced nuclear weapons program and sees nuclear weapons as a cornerstone of its national security.

Trump maintains that renouncing the Iran deal signals U.S. strength and toughness to North Korea. In his words, “I think it sends the right message.”

National security adviser John Bolton echoed these sentiments when arguing that by getting out of a bad deal with Iran, the United States sent “a very important message to North Korea that we’re not in these negotiations with them just to get a deal.”

Will toughness on Iran help Trump with North Korea? Here are three reasons to doubt it.

Leaders tend to focus only on the message they want to send

International relations scholars have long been interested in how countries can show “resolve.” There is a lot of debate over whether resolve matters, whether countries or their leaders best convey it, and how they should try to signal it.

Still, it’s important to keep the challenges of signaling in mind. While leaders usually think they are signaling toughness, they often don’t realize that their words and behavior may be sending several conflicting messages. During the Vietnam War, for example, both the Johnson and Nixon administrations tried to use aerial bombing to show Hanoi U.S. resolve.

But such messages required that Hanoi ignore the many other messages sent — by U.S. domestic protests, the state of combat in South Vietnam, and, for that matter, the administration’s need to send messages rather than impose a solution.

So, yes, Trump sent a “strong” message when he assailed the North Korean nuclear program with fiery rhetoric, backed by military demonstrations. Still, he shifted his rhetorical tone dramatically with early signs that North Korea was willing to discuss the “denuclearization” of the Korean Peninsula — a term that means something different to the North Koreans.

Trump is actually sending a number of incompatible messages. He backed out of the Iran deal, criticizing it for failing to control Iran’s non-nuclear activities and for permitting Iran to build up its nuclear capability in the future. But he didn’t explicitly challenge terms of the deal that allowed Iran to retain its nuclear infrastructure. Can North Korea reasonably assume, then, that a “denuclearized” North Korea could still possess a robust nuclear program?

For that matter, Trump didn’t just criticize the Iran deal. It was one in a long line of prior agreements he dubbed the “worst deal ever.” And Trump basked in the prospect of a potential summit and the possibility of making a great deal. The North Koreans may assume, then, that Trump will lower the bar to get his deal.

North Korea will view the wisdom of any deal through its own lens — and U.S. resolve might not matter much

Many U.S. presidents have obsessed over the global effects on apparent U.S. resolve should the United States fail to act decisively or stick to its guns. In Iraq and Vietnam, for example, they feared the United States would lose credibility and acquire a reputation for irresoluteness for not having followed through on a commitment.

But other world leaders see events through a prism of their own assumptions about how foreign governments are predisposed to behave. The U.S. departure from Vietnam did not have the effect that U.S. presidents had feared. Other countries drew their own conclusions about the meaning of the U.S. exit. Whatever message U.S. leaders seek to convey, then, North Korean assumptions about U.S. thinking will drive the North Korean approach to negotiations.

Consider North Korea’s heated response to John Bolton’s recent invocation of the “Libyan model” for North Korea. Bolton was apparently referring to former Libyan leader Moammar Gaddafi’s willingness to give up his weapons of mass destruction — openly and completely — to placate the West. But North Korea seemed more impressed by the consequences. A few short years later, the United States, Britain and France took up arms to help remove Gaddafi from power. If North Korea views denuclearization as a trick to make it vulnerable, U.S. resolve, no matter how it is signaled, will not produce a deal.

Policymakers focus on resolve at the expense of trust

International agreements inevitably require trust. Placing your fate in the hands of another country — here, the United States — requires quite a bit of faith that the country will keep its word. The agreement with Iran required its leaders to trust that the other parties would stick to the deal. Iran’s leaders did not assume that the United States would discourage trade and investment with Iran through a lingering threat to reimpose sanctions. Or that the United States would reverse itself and reimpose sanctions once Iran had dismantled or frozen its nuclear capabilities. They were wrong. North Korea has undoubtedly noticed.

With a U.S. reputation for reneging on deals, can North Korea believe that any reliable deal is possible, short of capitulation to U.S. demands? After all, the United States didn’t just withdraw from the Iran agreement, it violated it.

For North Korea’s leaders to agree to total denuclearization, they must trust, for example, that the United States will stick to the deal so that any acquired information won’t be used subsequently to attack North Korea’s remaining nuclear facilities. They must also believe that the United States won’t exploit the country’s vulnerability to try to push for a new regime. Their security fears are unlikely to have been reduced by Trump’s recent suggestion that the Libyan model (as North Korea understands it) “would take place if we don’t make a deal.”

Even if its leaders feel secure, North Korea might seek to “lock in” U.S. commitments, with a significant U.S. down payment, before taking irreversible steps toward denuclearization. Such a requirement will only increase the difficulty of getting a deal.

James H. Lebovic is a professor of political science and international affairs at George Washington University.