The Belgian police force raided around 48 houses searching for Salafi Muslim extremists who are thought to be recruiting jihadists to fight in Syria. "Shariah for Belgium" (although disbanded) is behind the recruiting efforts. Dozens of teens have already left to fight. Shariah for Belgium's aim is to overthrow the Belgian government and establish an Islamic caliphate there.
500 EU nationals are thought to be fighting in Syria, or between 8 and 10% of the rebels forces.
What the government fears is that these youth will return to Belgium- after being trained and radicalized in Syria- ready, willing and able to harm the Belgian people. Maybe they should just not allow them back in the country.