If you look solely at the U.S.’s long record of taking in refugees from countries torn apart by war, it’s hard to argue that national security should be a top concern in the debate over Syrian migrants.

In the 14 years since September 11, 2001, the United States has resettled 784,000 refugees from around the world, according to data from the Migration Policy Institute, a D.C. think tank. And within that population, three people have been arrested for activities related to terrorism. None of them were close to executing an attack inside the U.S., and two of the men were caught trying to leave the country to join terrorist groups overseas.

“I think I can count on one hand the number of crimes of any significance that I've heard have been committed by refugees,” said Lavinia Limón, a veteran of refugee work since 1975 and the president of the U.S. Committee for Refugees and Immigrants. “It just hasn’t been an issue.”

Yet it is the issue now, as the Obama administration tries to fend off a revolt by Republican governors over its plans to resettle more than 10,000 Syrian refugees escaping the brutality of both the Islamic State and the Assad government. The coordinated attacks in Paris have fanned fears that terrorists could infiltrate the U.S. by slipping in among the refugees—as might have occurred in the case of one of the Paris attackers.

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.

A central question that Republicans have raised is whether the U.S. has good enough intelligence and data from Syria to determine if a refugee might pose a threat. How extensive is their database? How easy would it be for an applicant to use forged or stolen documents to get into the U.S.? Critics of the refugee policy have gained ammunition from FBI Director James Comey, who acknowledged while testifying before Congress in October that there were “certain gaps...in the data available to us.” He declined to detail those concerns in an open hearing, saying he did not want to provide a roadmap for terrorists. “There is risk associated with bringing anybody in from the outside, but especially from a conflict zone like that,” Comey said.

Republican governors and congressional leaders (along with a few Democrats) have seized on those remarks in calling for “a pause” in the Syrian refugee program so it can undergo another review, and the House could pass legislation to that effect in the next few days. Refugee advocates, however, say there is little cause for concern. “I just don’t find that argument plausible,” Newland told me. She said the U.S. might have less data on Syria than on Iraq and Afghanistan, where the military has had a presence for more than a decade. “But I don't think there’s less information than there would be any other refugee population,” Newland said. She added that coming from a police state that likes to keep track of its people, refugees from “a well-organized society” like Syria would be more likely to have documentation than those fleeing from impoverished countries where citizens are unlikely to have government-issued birth certificates or passports.

Steven Camarota, the director of research at the right-leaning Center for Immigration Studies, said the key difference between Syria and most other sites of recent humanitarian crises is the heavy influence of a group devoted to the destruction of the U.S. and Western society. He also disputed the blemish-free history that advocates of the refugee program have clung to, citing Somali immigrants in Minnesota who have left the country to join ISIS and the case of the Boston Marathon bombers, who arrived as children after being granted asylum. Yet the process for receiving asylum status is not as stringent as for those applying for refugee resettlement, and those cases all involved people radicalized while they were living in the U.S. “The point here is,” Camarota said, “is it worth the risk?”

Immigration of any kind has caused tension and in many cases outright hostility throughout U.S. history, and refugee crises are no exception. In a 1939 poll recirculated widely on Tuesday, more than three out of five Americans opposed the resettlement of 10,000 Jewish refugees fleeing Nazi Germany. Oftentimes, the concerns have been economic. In the late 1970s, Newland said, fishermen in California feared competition from Vietnamese refugees who would be willing to work longer hours for lower pay than they did. And states and cities have occasionally asked the federal government to steer refugees elsewhere if they didn’t think they'd be able to find jobs in their communities. But the terrorism-fueled fears that have prompted a rush of opposition to Syrian resettlement is something else. “In my experience,” Newland said, “this is unique.”