In a speech Wednesday, President Trump announced a ban on travelers from Europe. This led to a lot of initial confusion over what Trump meant by “Europe,” and whether trade in goods was banned, too (Trump said it was, but the White House quickly clarified that this was not the case). Trump also said that the United Kingdom would be exempted from the ban, prompting a lot of speculation as to what exactly was going on. It turns out that the ban — and the exception for the United Kingdom — is not quite as arbitrary as it sounds. Trump’s ban extends to countries in what’s known as the “Schengen area,” a region that includes most states that are members of the European Union (as well as Norway, Iceland and Switzerland). The Schengen area does not include the U.K. nor Ireland, whose prime minister is meeting Trump Thursday for the traditional presentation of a bowl of shamrock for St. Patrick’s Day. So what is the Schengen area, and why has the Trump administration targeted it?

The Schengen area is an area of border-free travel

According to the Department of Homeland Security, the travel ban will apply to “most foreign nationals who have been in … the Schengen Area.” The idea behind the Schengen area is straightforward. It is supposed to allow people to cross national borders between Schengen members without having their passports or identity cards checked. This requires a lot of trust between countries, since, for example, once someone has entered Greece, they are in theory capable of moving to Germany, Italy, France, Hungary or any other member country without anyone stopping them or checking who they are. This has led to the creation of a massive system of information exchange among member countries. However, when Schengen was being created in the early 1990s, the U.K. decided not to participate. As an island state (spanning Britain and the northern part of the island of Ireland), it has a different understanding of borders than mainland European countries, and it was also suspicious of deeper integration with other European countries. Ireland had a common set of travel arrangements with the United Kingdom that it did not want to give up, so it too decided not to join Schengen (although the U.K. and Ireland shared information with Schengen after it became more closely integrated into the European Union).

The result is that it is possible, at least in principle, to travel across much of Western Europe without having your passport checked at the border.

Schengen did not really get rid of border controls

In practice, border-free travel was never quite as simple as the rhetoric suggested. States continued to maintain some control over their borders and conduct random spot checks. Recently, the politics of Schengen has become increasingly complicated, thanks to internal European disagreements about migration. When the Schengen area was created, no one expected that Europe would have to deal with massive flows of immigrants and refugees. Wars in Libya and Syria, and pressure from people who wanted a better life in Europe for themselves and their children, changed all that. The result was that Germany, Austria, Denmark, Sweden and Norway reintroduced border controls in 2015, to try to stop migrants who had landed in countries such as Italy and Greece from entering their territory.

The novel coronavirus outbreak has led to a similar reaction. The rapidly growing number of cases in Italy has led other Schengen members to talk about introducing new travel restrictions to prevent the spread of infection. Austria has said that people can only enter from Italy (which borders Austria) if they have a health certificate showing they are free from the coronavirus that is less than four days old. Slovenia and Hungary are introducing strong restrictions too. Such restrictions are probably legal under Schengen, which allows for the temporary reintroduction of border controls under exceptional circumstances.

But that isn’t good enough for Trump

Trump, in his speech, blamed Europe for not being careful enough in introducing restrictions, saying that viral clusters in the United States have been “seeded by travel from Europe.” The rationale for the ban appears to be that border-free travel will allow the contagion to spread throughout the Schengen area, and then jump to other countries such as the United States.

As political scientists such as Mara Pillinger have noted, governments are often enthusiastic to impose travel bans, even though there is a broad consensus among policy experts that they are costly, politically troublesome and not particularly effective. They are especially unlikely to be effective where a disease has already become endemic within a country, so that “community infection” is occurring.

Why then are some Schengen states restricting travel from other Schengen states, and why is Trump imposing a general ban on foreign nationals who have been in the Schengen area? One plausible explanation might be that travel bans send a highly visible signal to voters that the government is doing something to stop the spread. That might be particularly politically important in the United States, where the government has not yet been able to test citizens for the novel coronavirus in significant numbers, let alone introduce significant targeted measures to halt the spread of the virus that causes the disease covid-19.

Though many Schengen area states are likely to be very angry at Trump’s decision, the willingness of some of their fellow Schengen members to impose similar restrictions will make it hard for them to respond in an unified way. It will be particularly interesting to see whether states such as Austria and Hungary denounce the U.S. ban or seek an exemption on the basis that they are imposing restrictions, and whether Ireland will refer to the ban on its fellow E.U. member states in the prime minister’s meeting with Trump on Thursday.