In practice, however, multiculturalism tends to crate self-segregated communities, isolated along ethnic or religious lines. In France and Belgium, for example, migrants often inhabit areas where crime rates and anti-government sentiments are relatively high. The high birth rate among many migrants creates a heavy burden on their finances and the education system, and without a sound education, Muslim teenagers easily "fail at the starting line". This explains the high unemployment and crime rates among Muslims in Europe.

This divide between minority and majority communities will widen the socioeconomic gap and prompt the disadvantaged groups to question the authority of the state. In France and Belgium, Muslims are banned from covering their faces in public, but in many districts, say, the Molenbeek-Saint-Jean in Brussels, more and more Muslim women are voluntarily wearing the hijab or headscarves in public. Moreover, growing numbers of young Muslims are attending "underground" mosques that advocate jihadism. This further increases the conflicts or interests in multicultural Europe.

Since the end of the Cold War, freedom and democracy have occupied the core place in global politics; they are the touchstone of political correctness. But the blind pursuit of political correctness is compromising public security. Many Muslims see the burqa ban in France as "discriminatory". So if they demand "freedom of religion and expression" claiming the burqa is part of Islamic culture, how will the majority of Europeans, who swear by "cultural freedom" react?

The biggest challenge for European officials, therefore, is how to make Muslim migrants merge into modern democratic society, in order to end social fragmentations created by ethnic tensions. But for that, Europe cannot advocate freedom without principles.

Since terrorism threatens public security and human values, European decision-makers have to realize that "desperate diseases must have desperate remedies". In public policy, Europe should rethink its "welfare policy" and consider promoting a "work for living" rule among some communities. This is not to say that migrants' social welfare and security should be withdrawn. To cleanse the soil that breeds terrorism, European governments should also take intensive measures to spread modern values among the migrant population.

The author is with the Faculty of Law and Criminology of Vrije Universiteit Brussel.