Timothy Stanley is a historian and columnist for Britain's Daily Telegraph. He is the author of "Citizen Hollywood: How the Collaboration Between LA and DC Revolutionized American Politics." The opinions expressed in this commentary are his.

(CNN) Donald Trump pulling America out of the Iran nuclear deal is the biggest thing he's yet done on the international stage, with the greatest consequences. I understand entirely why people might feel nervous: Iran's warning that it is ready to start nuclear enrichment was frightening. But the President is, on balance, correct. Either the world deals with Iran now, or it deals with a nuclear Iran in the future.

The argument for keeping the deal going was reasonable: no one advocating the status quo was "weak" or trying to preserve their Obama-era legacy. But the deal amounted to bribing Iran to suspend its nuclear program, which is like feeding a wild animal scraps of meat to dissuade it from biting you. The deal lifted sanctions, threw the regime an economic lifeline and effectively recognized it as part of the international order. It moved toward the regularization of a pariah state.

The problem is that Iran's regime is not regular. Look at what it's been doing since the deal was signed. Even if uranium enrichment has been stopped, Tehran has continued, quite openly, to invest in its missile program. It has harassed foreign citizens

It has also hugely expanded its power within the region, establishing a presence in Yemen, Syria and Lebanon. Indeed, a country's attitude toward the Iran deal depends in part on its proximity to Iran. Israel, for instance, cannot tolerate its neighbors being turned into military colonies of a hostile foreign power. It feels vulnerable to attack.

If the deal stands as it is, there is a risk that it actually enables the emergence of Iran as a hegemonic power on the brink of attaining the bomb. It would have the same effect as the SALT treaties of the 70s, which were meant to limit Soviet arms growth, but in fact provided cover for the Soviets expanding into Africa, Central America and Asia.

It was the collapse of SALT and the early 80s arms race that really pushed the Soviets to the negotiating table, not détente -- and the same logic, I suspect, is applied by the Trump administration to Iran. Why give Iran money, and thus stability, just as its economy is teetering and its people are demonstrating in the streets?

Confrontation with Iran is, for its neighbors at least, inevitable and more or less already happening. Trump's decision to draw a line by pulling out of the Iran deal brings in two new dynamics. First, it shifts the focus of present US policy away from just tackling Islamist terrorism that tends to be Sunni (the Islamic State), and toward identifying Iran as a key destabilizer of the Middle East.

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Second, with Trump's decision, the center of gravity in policymaking shifts. The Europeans -- principally Germany, France and Britain -- staked everything on saving the deal. Their diplomatic failure was humiliating. Instead, momentum is now with the United States, Israel and Saudi Arabia. Of these, the most unreliable partner is viewed as Saudi: the United States is allying with a religious dictatorship to contain a religious dictatorship. Life is full of surprises. Trump was elected, many thought, to get America out of the Middle East. Instead, he's helped forge a new local coalition and backed it against Iran.

Let's be frank about the long-term goal. The United States wants regime change. Trump's advisers calculate that Iran can never be dealt with as a normal country, that it is constitutionally opposed to US interests, and that it is ripe for revolution. I don't know if all of that is true. I cannot say chaos won't follow this decision. But it had to happen.