One of the lines Iranian diplomats and supporters like to repeat is that the Islamic Republic will not change its behavior in response to pressure. Sanctions and threats don't work, they say; engagement and mutual respect do.

This principle was best illustrated in a particularly testy moment during the final days of the 2015 nuclear negotiations. Western foreign ministers were trying to keep in place a U.N. conventional arms embargo on Iran, and they brought up the regime's support for terrorism throughout the Middle East. Iran's foreign minister, Javad Zarif, responded that he could bring American and European governments before The Hague for their support of Saddam Hussein in the 1980s. "Never threaten an Iranian," he said, according to multiple reports at the time.

It's true that sanctions against Iran really began in the mid-1990s, but it was not until 2011 and 2012 that crippling sanctions cut off the country's oil revenue and its ability to participate in the international banking system. It was those sanctions that forced Iran - facing a cash crunch - into negotiations in 2013, which ultimately produced the nuclear deal.

Michael Rubin, an Iran expert at the American Enterprise Institute and the author of "Dancing With the Devil," a history of diplomatic engagement with rogue regimes and terrorist groups, told me, "If you want to make Iran change its position, you have to make their isolation too great to bear." He said one of the reasons Iran expanded its centrifuges before 2013 was because between 1998 and 2005 European trade nearly tripled with Iran while the price of oil spiked.

Iran has also historically responded to explicit and implied military threats. The most famous example involves its nuclear weapons program. Most of the regime's work to design and test a nuclear weapon was conducted before 2003. Why did Tehran stop that year? Rubin said the U.S. invasion of Iraq under George W. Bush was one factor in the regime's thinking.

The inverse is also true. When the Iranian regime feels secure, it gets more aggressive. After Iran agreed in 2015 to the nuclear agreement, the commander of its Quds Force, Qassem Suleimani, traveled to Moscow and arranged for Russia to provide air support for Iranian-backed militias fighting for the dictatorship in Syria. It also began testing ballistic missiles.

In the days before the implementation of the nuclear deal, Iran's Revolutionary Guard Corps boarded a U.S. Navy vessel and briefly detained U.S. sailors who had accidentally drifted into Iranian waters. In case the point was missed, the sailors were videotaped on their knees with their hands clasped behind their heads.

Iran's aggressive maneuvers in the Persian Gulf against U.S. vessels have decreased since Trump came into office. Analysts have argued there could be multiple reasons for this change in behavior. One reason though is an assessment by Tehran that Trump - the man who promised during the campaign to blow Iranian vessels out of the water if they came too close to U.S. ships - might follow through on his promise.

It's too soon to say whether Trump's gamble this week to exit the Iran deal will end up creating a nuclear crisis. As Rubin told me, "I don't know if the Iranians know what they are going to do yet." He said one of Iran's strategies is to divide America from its European allies. The U.S. decision to walk away from the Iran deal may present such an opportunity.

For now though it's worth lingering on a line in Trump's speech this week when he announced the U.S. withdrawal from the nuclear deal: "If the regime continues its nuclear aspirations, it will have bigger problems than it has ever had before." For the sake of the world and the region, let's hope Iran's leaders find that threat credible.


Eli Lake is a Bloomberg Opinion columnist.