This weekend, Spain and Germany arrested 11 members of the armed terrorist organisation ISIL, the group that has taken over large swaths of Iraq in recent days. The leader of the Spanish cell, from which eight of the arrests were made, is a former Guantánamo detainee. The ISIS members in Spain were allegedly working to recruit new members to fight in the organization’s broad jihadi campaign to establish a neo-Caliphate in Iraq and Syria. One of the suspects arrested in Germany allegedly has fought with militants in Syria and appeared in ISIS propaganda videos.This isn’t the first spillover of violent extremists from the current conflict in the Middle East into Europe: On June 1, a French citizen suspected of being a Jihadist went on a shooting spree at a Jewish Museum in Brussels after returning from Syria, killing three visitors. And it’s not the first indication that European and U.S. authorities have concerns about spillover effects—they issued grave warnings late last year.The fateful choice not to do anything in the ongoing chaos in Syria has already had many costs, both humanitarian and strategic. And many more of these costs are likely to accrue in the future.