The government of the Netherlands raised its terror threat level to "substantial" today, amid fears that terrorists trained in Syria will try to disrupt the coronation of their new king. The head of Dutch counterterrorism says that as many as 100 Dutch citizens have traveled to Syria to join in the war there, and are returning angry, radicalized, and highly-trained to wreak havoc on Europe. He also cited an increased radicalization among Dutch youth in general, making them highly susceptible to recruitment by these returning jihadist fighters.

The government also warned that the Netherlands is not the only country that has to worry about this problem. With the Iraq and Afghanistan wars on the wane, Syria has become perhaps the most active proving ground for new terrorists, who have found freedom of movement and a sympathetic population grateful for the help they've given to take down a brutal dictator. They also gain knowledge about things like weapons, bomb making, communications, and guerrilla fighting tactics. Thousands of foreign fighters have joined the anti-Assad cause, and when (or if) that war ever comes to a conclusion, many of them will look to join a new fight elsewhere. It could be Yemen or North Africa, but it could also be the cities of Europe that many of them used to call home.

The Dutch have also had a fraught history with Islamic radicalism, and with the their large population of immigrants from predominantly Muslim countries. Several Dutch politicians have made names for themselves—and found surprising success at the polls—by criticizing Islam, and pushing anti-Immigration laws designed to keep Muslim populations from growing. (Two of them even made a list of al-Qaeda's "most wanted" blasphemers.) In 2004, Theo van Gogh, a prominent documentary filmmaker, was murdered in broad daylight for making a film that was critical of the religion. Earlier this year, the government moved to ban burqas and face veils and just this week a new controversy arose after video surfaced of young Muslim praising Hitler and saying he "should have killed all the Jews.Numerous studies have found that young Muslims feel alienated and resentful of the society that can't seem to accept them—which leads to distrust from their fellow citizens, more resentment, and a continuing the cycle of hatred. Adding the violent history of Syria to that mix could create a very dangerous situation.

Obviously, any concerns about security are going to ratcheted up next month when the Netherlands installs its new monarch. Queen Beatrix announced in January that she would abdicate the throne on April 30, turning the crown over to her eldest son, Willem-Alexander. The upcoming royal celebration is sure to be wild and crazy time for Dutch citizens (Amsterdam's hotels are already sold out), but a particularly trying time for those trying to keep them safe.