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President Donald Trump's multi-pronged campaign to address the potential threats posed by refugees and migrants has a familiar ring. New border barriers with Mexico, a sharp reduction in refugee admissions, a crackdown on so-called "sanctuary cities," and new restrictions on travel from countries thought to pose a terrorist threat—all these and more were part of the U.S. reaction to the September 11 terrorist attacks.

But there is one critical difference this time around. In the aftermath of 9/11, the United States had a president, George W. Bush, who believed that America's strength came from its openness to immigrants and to trade. His administration closed many doors in the name of homeland security, but President Bush was eager to open them again as soon as it seemed safe to do so.

Donald Trump is no George Bush. As the new president said in his inaugural address: "Protection will lead to prosperity and strength." Unlike President Bush, Trump's goal is not to implement the least disruptive security measures consistent with the U.S. tradition of openness, but rather to invoke security to shut the United States off to the greatest extent possible. For Bush, the walls built to Mexico under his watch were a necessary deviation from his principles; for Trump, the wall will be a proud symbol of a very different set of principles.

Trump's announcements are far from a fait accompli, of course. He needs money from Congress to build the wall; big cities like Los Angeles and New York will fight him at every step to prevent deportation of illegal migrants who have not committed other crimes; the State Department and possibly even the Department of Homeland Security will fight against the travel restrictions. American universities, which saw a drop in foreign students after 9/11, will push back, and they have the ear of virtually every member of Congress. The travel industry—the airlines, hotels, and restaurants that benefit from foreign tourists—will also object. The new DHS Secretary General John Kelly expressed skepticism at his confirmation hearing about targeting Muslim nations with new travel and immigration restrictions, and may be a reluctant ally for Trump. Foreign governments will complain vociferously; incoming Secretary of State Rex Tillerson may find many of his working days consumed with soothing hurt feelings around the world.

In my book about the post-9/11 years, The Closing of the American Border, I documented the fierce internal struggles in the Bush administration over the new security measures. There were pitched battles between the State and Justice departments for example, with Secretary of State Colin Powell resisting many of the restrictions being pushed by Attorney-General John Ashcroft. State lost many of the early battles, but ended up winning the war. By the end of the Bush administration, most of the restrictive measures had been removed.

This will play out differently under Trump. Unlike the immediate aftermath of 9/11, Trump is responding to a crisis mostly of his own imagining. The new measures are being rolled out enthusiastically, not reluctantly. The White House will look for opportunities—such as any new terrorist attacks overseas—to extend additional restrictions.

That argues for a very different set of tactics for opponents of Trump's border and immigration strategy. They cannot count on a supportive president who will work to roll back restrictions as the public's sense of crisis wanes. Instead, the president himself will be sowing fear to create a crisis environment that invites still more restrictions.

For those committed to American openness—who believe it has been among the greatest sources of American strength—this will be the biggest test in generations. It is a battle that will be much harder to fight without an ally in the Oval Office.