Because of the involvement of these Dutch citizens, the Netherlands has upped the terror threat for the country from "limited" to "substantial".
"The number of those jihadists travelling from the Netherlands to Syria has rapidly increased," the National Counterterrorist and Safety Coordinator (NCTV) said in a statement.
"There is a risk that those who travel for jihad pick up experience in conflict areas and upon return to the Netherlands form a threat because they could come back radicalised, traumatised and willing to commit violence," the government's anti-terror body said.
The threat of an attack against the Netherlands or its overseas interests has grown, prompting the NCTV "to increase the threat level from limited to substantial," it added.
Read the rest here.
The Dutch aren't the only ones frightened about the return of radicalised fighters, so is France and Germany, and I would think England should be just as worried.