Shortly after the tragic terror attacks that occurred in Paris the evening of November 13, French President Francois Hollande ordered the closing of his country’s borders to prevent anyone involved in the attacks from leaving France. However, carrying out such an order by bringing all cross-border traffic to a halt—even in a country as relatively small as France—is easier said than done.

France has a 1,274-mile long land border along its eastern edge that it shares with six countries: Belgium, Luxembourg, Germany, Switzerland, Italy, and Monaco. To the south, it shares a 436-mile border with Spain and the tiny nation of Andorra. But what makes border security after a crisis so challenging for a country like France is the fact it’s a member of the European Union (EU). This renders more traditional border crossings and checkpoints obsolete since citizens of EU countries can travel freely between them without having to show any identification or use a passport.

However, if you are not an EU citizen, you do have to carry a passport. Border officials in EU countries may ask for other supporting documents such as an invitation letter, proof of lodging, return, or round-trip ticket. You also have to provide your passport when checking into a hotel. If the perpetrators of the attacks in Paris are EU citizens and their identities are unknown to law enforcement or not on a terrorist watch list, then typically they would have no problem crossing into any of these neighboring countries.

To truly shut down its borders and prevent any suspects from fleeing, France would have to close down dozens of airports (considered a port-of-entry and part of a border security strategy), prevent all vessels from entering and leaving its 12 maritime ports, stop all Eurostar trains that travel between France and the United Kingdom, and beef up security at the six railway stations France considers land border crossings. It would also have to coordinate with agencies responsible for manning the dozens of border crossings in the six nations to the east.

France has two agencies responsible for monitoring border crossings: its Customs service, which is responsible for customs checks akin to US Customs and Border Protection, and its Police aux Frontières, or Border Police, who handle French immigration checks and conduct border surveillance in some areas of France. According to TripAdvisor:

Land border checks in the EU exist, but are usually targeted rather than general. The vast majority of land crossings from other Schengen countries will encounter no checks whatsoever. Border control is maintained with Andorra, owing to the tax haven status and low duty and value-added tax on purchases. The UK also maintains border control with France, at train and ferry terminals, as the UK opted out of the common border control part of the Schengen Agreement.

France has experienced its share of border control problems as waves of refugees fleeing conflicts in Syria and Africa have flooded several EU nations. In June 2015, Italian police had to forcibly remove several migrants who had been camped on the French border for days after Paris tightened security measures to prevent them from crossing. According to the International Business Times:

Struggling to cope with the thousands of asylum seekers who land on its shores from North Africa, Italy has been pressuring its European allies to endorse a common refugee redistribution plan to alleviate pressure on its overflowing reception centres. This has met the resistance of other European countries including France, which has stepped up border controls, drawing the anger of Prime Minister Matteo Renzi, who accused Paris of selfishness.

Earlier in June, French riot police stormed a migrant camp in Paris and marched everyone onto buses set for hostels. Many migrants like these are headed towards the UK. Official figures released at that time indicated French police stop migrants trying to sneak into Britain a staggering 130 times a day, most of them trying to use either a ferry service or the English channel tunnel.

So while French immigration and customs authorities have already been clamping down on border controls since the flood of migrants from Syria and Africa began, it’s clear that these measures have not been sufficient to keep all migrants from either coming or going. Even in the face of crisis when a nation’s citizens—and indeed the world—are more accommodating to stringent security measures, Hollande’s order to shut down France’s borders may be an order too tall for his country to fill.

Sylvia Longmire is a border security expert and Contributing Editor for Breitbart Texas. You can read more about cross-border issues in her latest book, Border Insecurity: Why Big Money, Fences, and Drones Aren’t Making Us Safer.