As U.S. officials and refugee advocates point out, that has never happened in modern history. Not when the U.S. took in tens of thousands of Vietnamese refugees in the 1970s. Not when 125,000 Cuban “Marielitos” arrived by boat in 1980. And not in the desperate aftermath of more recent wars in Bosnia, Somalia, or Rwanda. “Those fears have proven unfounded,” said John Sandweg, a former acting director of ICE who previously served as a top lawyer at the Department of Homeland Security.

Is there any reason why Syria should be different?

The government and the nonprofit organizations it partners with to resettle refugees cite two main reasons why the answer is no. The first is that there is a key difference between people seeking placement in the U.S. as refugees and the millions of people who have flooded into Europe seeking asylum. The Syrians in Europe in many cases are already at or over the border, having come directly from Syria in to Turkey and then Greece and elsewhere; that situation is more akin to the thousands of Cubans who have fled by boat to South Florida or the migrant workers from Central America who gathered at the U.S.-Mexico border last summer. A refugee applying for resettlement in the U.S., by contrast, must endure a screening process that takes as long as two years before stepping foot on American soil. “Germany doesn’t have the luxury of screening them or vetting them in any way before they arrive, unlike the United States,” said Kathleen Newland, a senior fellow and co-founder of the Migration Policy Institute.

The second reason is that since the program was briefly halted and then overhauled after the 9/11 attacks, refugee applicants are subject to the highest level of security checks of any type of traveler to the U.S. The United Nations High Commissioner for Refugees initially chooses which refugees to refer to the U.S. after doing its own check. U.S. officials then conduct multiple in-person interviews and verify a refugee’s story with intelligence agencies and by running background checks through several government databases, including DHS and the National Counterterrorism Center. As a result of that extensive process, only around 2,000 Syrian refugees have been resettled in the U.S. since its civil war broke out in 2011—a much lower number than many previous refugee crises. The Obama administration wants to accept at least 10,000 more in 2016, but even that might be too much for the bureaucracy to handle. Once resettled, refugees get housing and monetary assistance for several months. After a year, they can apply for a green card, at which point they undergo another security screening.

More than half of the nation's governors—mostly Republicans—are now urging the federal government to keep Syrian refugees out of their states. But they probably don’t have the final say. Courts have ruled that immigration policy is almost entirely a federal matter, and while the Obama administration says it must “consult” with states as part of the refugee program, the states can’t reject immigrants entirely. Yet as a practical matter, because the benefits that refugees receive are administered at the state level, the government might be unlikely to send them to states where they won’t be welcome.