There are reports that more than 31,000 people have fled the recent fighting in the city. Sadly, among the genuinely desperate and dispossessed, there may well be IS soldiers who believe that their best chance of continuing the fight lies not in the city of Aleppo but in the cities of Europe.
These are the people that Europol are warning about. It is as well to have our eyes open about the threat that faces us.
The European police force rightly warns that the threat has altered now and that it is likely to be lone attackers working with blades and vehicles as weapons. That makes them difficult to find in advance of any attack, and even those weapons can be horribly powerful as was evident from the Nice lorry attack.
But what history tells us is that people in war zones have easy and unrestricted access to much deadlier weapons, and it is easy to leave the battlefield with automatic weapons and grenades and mines and shells. It is even easier to take away the knowledge of how to make improvised explosive devices and transfer that to an urban arena.
While improved surveillance techniques, allied with enthusiastic inter-agency co-operation, can play a vital part in protecting our populations from any attack, if we are all aware of the potential threats, then we can all play a part strengthening that protection by being ever vigilant.