Neither French President Emmanuel Macron nor German Chancellor Angela Merkel visited President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE last week with high hopes of changing his mind about the Iran nuclear agreement. But both European leaders no doubt reminded the American president that walking away from what he derides as the “worst deal ever” will not be so easy.

In foreign policy, every action has a reaction, and, as in a gentlemen’s duel to settle a disagreement, Trump may fire first, but Europe can and will fire back. Given transatlantic interconnectedness, the ramifications and reverberations of Trump’s pending Iran deal decision this month will be felt not just in Europe, but at home in Washington. Trump should tread carefully: Europe has its own hand to play.

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Make no mistake, if Trump upends the Iran nuclear agreement while Tehran is in compliance, he will bring about the single biggest alienation of the United States from its European allies since President Bush’s invasion of Iraq in 2003. It took our country seven years to rebuild the prestige and credibility we lost on the world stage because of that misadventure. Trump’s abrogation of the world’s work of more than a decade to rid ourselves of the Iranian nuclear threat would represent even more of a seismic shift away from our allies of first resort.

Germany’s foreign minister says the Iran nuclear agreement “prevented a nuclear arms race in the Middle East.” The International Atomic Energy Agency in Austria is charged with monitoring Iran’s activities confirms that Iran is in compliance with the toughest standards ever agreed to in a nonproliferation framework. Trump could let the Iranian nuclear genie out of the bottle at great peril to his European relationships. Perhaps to Trump, these European alliances aren’t of great consequence, but they should be. It is, after all, this president who has insisted that Europe share the burdens of national security in greater proportions. Alienating Europe is a way to ensure the opposite.

When I served as U.S. ambassador to Italy during President Obama’s administration, I saw just how much the security of the United States was connected to our diplomatic and military cooperation with Italy. We cooperated on basing and joint exercises and sorties for Libya airstrikes out of Sigonella. We shared the burden on efforts to provide humanitarian help to Syria. Italy was an effective convener for global efforts to support struggling nations after the Arab Spring roiled the Middle East and North Africa. Driving away our allies is a guaranteed way for the United States to have the burden for essential efforts land squarely in our lap, and to lose the ability to pull allies to our side.

Trump may not appreciate how vital these relationships are, but as a native New Yorker, he should. NATO’s Article V commitments were invoked, for the first and only time, to come to America’s aid after the 9/11 attacks. European troops have deployed side by side with ours in Iraq and Afghanistan. Europe may not be able to persuade Trump to pay heed to common sense but can certainly pressure him with the reality of transatlantic leverage.

First, as Merkel has said, a consequence of Trump splitting the United States away from Europe on Iran could be that it pushes our closest allies toward more constructive and more necessary relationships with Russia, China and Iran itself, all of which want the nuclear agreement to remain intact. This would be a tough pill for the United States to swallow and is likely to jeopardize American influence over many important questions, from western Europe’s energy independence from Russia to access to the Arctic region and freedom of navigation in the South China Sea.

Second, Europe has more leverage than Trump concedes to keep the deal afloat without us. Would Washington really enforce secondary sanctions on European companies? Would Brussels block them? Would this incentivize a shift away from conducting business through U.S. channels? While the economic impact is likely to be minimal given that even after the Iran deal, Europe’s trade with Iran is a minimal percentage of its economies, but the opportunity to set precedent that undermines the preeminence of our financial institutions is undeniable.

Europe could take the United States to the woodshed at the World Trade Organization. Europe has precedence for bucking American ideological bombast, as just before this century, Brussels launched a case against the United States over American penalties against foreign companies trading with Cuba, and it worked. Compromise was struck and the United States largely surrendered the threat of secondary sanctions for continuing to do business with Cuba.

What I know is true on both sides of the Atlantic is to expect a fight. Europe fought hard and sacrificed much in order to arrive at a nuclear agreement with Iran that protected a continent that is geographically, culturally and historically closer to the Middle East than the United States will ever be. This issue is deeply personal to Europe. President Trump risks finding out just how personal it is the hard way.

David Thorne served as U.S. ambassador to Italy from 2009 to 2013.