Shutting out refugees is easy. Trump must also stop illegal immigration.

This week’s tragedy in Berlin appears to have been a terrorist act, perpetrated by a Tunisian national with a shady past. Apparent bungling by German and other European police and immigration services allowed the perpetrator to avoid extradition, carry out the crime and, at least for the present, elude arrest.

The inevitable sequel was a rapid media refocus on which immigration policies President-elect Donald Trump will pursue as he strives to protect the United States from terrorist events that have bedeviled European politics and law enforcement in recent years.

The recent flood of Asian and African immigrants escaping wars and political upheaval has intensified existing differences between native Europeans and immigrants, and created a massive pool of humanity that is too great for European security services to sift in their search for miscreants. American liberals, media and Democratic lawmakers fear that the United States could decide to short-circuit the problem by not letting refugees into the country in the first place.

Many people on the other side of the political spectrum fear just the opposite — that Trump entertained them with promises of slamming the door on Muslim immigrants and now seems to be softening, proposing instead to admit them after a process of “extreme vetting.” The idea of admitting thousands of unvetted aliens may be frightening, but it is merely one aspect of a problem that is stunning in both complexity and consequence.

{mosads}Attacks like 9/11 demonstrate how consequences can exceed events that precipitate them. Cities like New York actually go back to business pretty much as usual with surprising speed. The consequences are another matter.

As tragic as the personal losses were on 9/11, the United States has seen far more lives ruined avenging those losses, and the monetary cost has run to the trillions. Why invite more of the same? If there are problems in Syria, they should be settled in Syria, not brought here. Going down to the local mission to feed the poor and homeless on Christmas is one thing; bringing them all home to Mom is quite another.

But that’s the easy part. America faces a morass of complexity connected to illegal immigration that has already occurred, most of it from south of the border. The infusion of politics and skewing of mainstream media have made it impossible to discuss immigration objectively. People who favor helping immigrants are considered mush-headed morons; those who oppose are Neanderthals and Nazis.

Nonetheless, Trump should ignore labels and attack certain problems.

The Sanctuary City concept should be the first to go. The idea that mayors, and even some police chiefs, have decided to shield criminal illegals from deportation is not only counter to U.S. Code, but has the foul odor of nullification of federal law as practiced by Southern states during the immediate antebellum period of our history.

Trump would be well within his authority to order the detention of Mayor Bill de Blasio (D) of New York or Mayor Rahm Emanuel (D) of Chicago for violating federal statutes as an object lesson to the rest. Both of these men should be taught to pay a little more attention to the Americans in their cities who have critical need of the resources that are diverted to various programs for illegals.

Trump needs to phase out President Obama’s Dream Act, which allows young people who were brought to the United States by their illegal immigrant parents to be exempt from deportation. The argument that these young people have never known another home is reasonable, but this does not explain how their families get to flout immigration law.

For example, Dreamers in college or the military could stay. Kids go away to college all the time. They would be made eligible for citizenship, but would have to make a decision to stay here and make a new life or go home to their families. Tied to this should be a constitutional amendment that does away with automatic citizenship for children born in the United States to illegal immigrants.

And don’t expect the debate on these issues to ever be reasonable. The likelihood is that immigrants will vote Democratic. This ties directly to doing away with the Electoral College. Democrats want presidential elections to be decided by popular vote in a few border states progressively tilted by legal immigration, and by illegal immigration tied to a path to citizenship.

These are many other issues related to immigration, but two principles should apply regardless: Illegal immigration is a crime, and legal immigrants must be certified to be unlikely to endanger us.

We do a lot for our residents. The least they can do is obey our laws and refrain from doing us harm.

Joseph Blady, M.D., is a former program officer for the undersecretary of Defense for policy and senior analyst for the undersecretary of Defense for intelligence.

The views expressed by contributors are their own and not the views of The Hill.