In September, the president of the European Commission, Jean-Claude Juncker, called the free movement under the Schengen Agreement “a unique symbol of European integration.”

But what once seemed a sensible idea now offers real and present danger. Stolen, doctored and fake passports from the Schengen area are among the most sought-after forms of identification by terrorists, drug smugglers, human traffickers and other criminals. As of last year, eight Schengen countries were on the list of the top 10 nations reporting stolen or lost passports in Interpol’s databases. Not one of those countries systematically screened passports at their borders.

Among the European countries that are not parties to the Schengen Agreement is the United Kingdom, which began screening passports against Interpol’s database following the 2005 terrorist attacks there that killed 52 people and injured more than 700. The U.K. now screens about 150 million passports a year, more than all other European Union nations combined, and catches more than 10,000 people a year trying to cross its borders using invalid travel documents…

Having open borders without the proper vetting aids and abets terrorists. The failure to thoroughly screen passports or check identities at border crossings is simply irresponsible in the face of global terrorism. Based on my 14 years of experience running Interpol, I know that terrorists will be much more likely to succeed as long as countries fail to properly check the identities of those who cross their borders.