When Gabriela left her home in El Salvador in 2011, she did not know anything about U.S. immigration law. She just knew that she needed to escape from domestic violence and find a new life elsewhere.

After spending weeks crossing Guatemala and Mexico, she arrived at the U.S. border, where she eventually learned she was eligible to apply for asylum. While she waited six years for her case to work its way through the system, she was able to live and work in the U.S. After she was granted asylum in 2017, she was able to apply for asylum for her two children as well.

Though their experience was long and difficult, Gabriela — whose name has been changed because she is worried about a pending green card application — says she is grateful that in the U.S. “if you work hard you’ll be able to take care of yourself and have a good life.”

Each year, thousands of migrants like Gabriela apply for asylum in the U.S., saying that they have been persecuted due to their race, religion, nationality, political opinions or membership in a social group. Now, President Donald Trump is seeking to dramatically remake that system.

After a recent surge in migrants fleeing violence and poor economic conditions in Guatemala, Honduras and El Salvador, Trump threatened tariffs on Mexico if it didn’t agree to take action to stem the flow of asylum-seekers arriving at the U.S. border.

Among the measures Trump is seeking is something called “safe third country,” a policy where migrants would need to apply for asylum in Mexico or other countries first. The Mexican government has already agreed to expand a program that requires thousands of migrants to wait in Mexico while the U.S. handles their applications, a process that can take months or even years.

Read More: President Trump Wants Asylum Seekers to Stay in Mexico. Here’s How That Would Work

Experts say the proposed changes would reverse decades of policy on how the U.S. treats people seeking asylum.

“What makes it sort of a stunning moment right now is the degree to which the Trump Administration is trying to remake — and that may be too soft of a word — the American asylum system,” says Carl Bon Tempo, a history professor at the State University of New York at Albany and author of Americans at the Gate: The United States and Refugees During the Cold War.

In order to apply for asylum in the United States, migrants must meet the definition of a refugee and be present in the U.S. This is different from applying for refugee status, when foreign nationals must secure permission before arriving at a U.S. border.

The United Nations defined who is considered a refugee at its 1951 convention. And though the United States was not a party to that convention, it signed on to the 1967 Protocol, which removed geographic and time limits on who could seek protection. Then with the Refugee Act of 1980, the U.S. brought these definitions into its own immigration laws.

So by requiring asylum seekers to wait outside the U.S. or by sending them back to whichever country they crossed first when fleeing their homes, the Trump Administration is fundamentally changing these definitions. Because the mainland U.S. shares borders only with Canada and Mexico, that would dramatically reduce the number of people who would be eligible for asylum.

“That’s an audacious move to try to almost destroy this category of asylum,” Bon Tempo says.

The United States has rejected those seeking refuge many times before. Immigration restrictions run through much of U.S. history, and even during the enormous refugee crisis around World War II, the U.S. denied many people entry. In one of the most infamous incidents, the U.S. in 1939 turned away a ship carrying nearly 1,000 Jewish refugees. The MS St. Louis was bringing Jews away from Germany and toward Cuba, but after they were denied entry in Cuba, the State Department told the ship that its passengers would have to join the years-long waiting list. The ship returned to Europe and more than 250 of the people onboard ultimately died in the Holocaust.

The U.S. is not alone in turning people away, however. The ideas that Trump has promoted are similar to how the European Union currently functions, according to Kathie Friedman, associate professor of international studies at the University of Washington’s Jackson School of International Studies. When it comes to E.U. countries, migrants are supposed to apply for asylum based on the first place they arrive. This has meant coastal nations like Italy, Spain and Greece have seen particularly high numbers of migrants arriving by boat from elsewhere in the Mediterranean.

That has led to rising tensions over how to handle migrants in Europe in recent years.

“Now we have this right-wing populist government in Italy because the rest of Europe didn’t share the obligation in taking care of asylum seekers,” Friedman says. “You get this kind of xenophobic reaction and hostility to refugees arriving on the coasts.”

In addition to the backlash against migrants, some countries cannot handle an influx of new residents. Greece, for example, has seen its already fragile economy strain under the increase, and it still houses tens of thousands of migrants in crowded camps that often have terrible conditions.

Sometimes it can work for countries that neighbor those in trouble to accept asylum seekers, such as after World War II when Jews and others who escaped the Nazis went to countries such as Sweden, England and Canada. Some even settled in West Germany after leaving displaced persons camps. But those countries had more resources and stability to support refugees than many countries in Central or South America do today, experts say.

“Requiring asylum-seekers to request asylum in the first country they transit through puts an enormous burden on the countries that border areas of crisis,” Maria Cristina Garcia, a Cornell University history professor and expert on migration, wrote in an email. “And these countries that border areas of crisis may be in crisis themselves. Their legal systems may not even recognize the right to asylum.”

Mexico, for its part, has already been detaining and deporting migrants, said Christopher Wilson, deputy director of the Wilson Center’s Mexico Institute. Mexico has even been receiving a growing number of asylum applications from Central American migrants. But the country’s asylum system needs help, and there are still safety concerns there for people fleeing violence. Mexico’s labor market has been fairly tight and could likely take in some migrants. Ironically, the tariffs Trump threatened — which are still a possibility in the future — would not help that.

“The tariffs themselves would be bad for both the U.S. and Mexican economies,” Wilson said. “It could limit the amount of budget that Mexico has to focus on enforcement, on its asylum system, and other areas where they really need to be investing more not less.”

For now, migrants coming to the U.S. through Mexico will still have to wait south of the border even if they apply for asylum. For Gabriela, being sent home or forced to wait in Mexico for six years would have been devastating.

“I don’t even want to think about what would have happened,” she says. In addition to navigating the asylum paperwork, she notes that it costs a lot of money for migrants to pay guides to help them reach the border. If she were turned back at that point, that would have left her worse than she started.

Now, Gabriela lives in a house with her new partner and two children. Her kids, now 11 and 15, are enjoying their new school and Gabriela was able to apply for a loan and attend cosmetology school this year.

“I really can’t explain how good it feels,” she says, “how much of a relief it is to be able to live here, to be free and to be able to enjoy my children — and to not fear the danger that we would be in if we were still back home.”

Write to Abigail Abrams at abigail.abrams@time.com.