Terrorist acts, after the initial shock and disgust are all about creating uncertainty, fear and disruption to our lives. That's why the people of Paris were fantastic to publicly unite in the way that the did.
Of course, all major UK cities will now wonder who is next and when, and in that sense, the Terrorists have achieved something.
The biggest worry is despite the amount of efforts into making travel, particularly airline travel as safe as possible, these attacks can easily be random and in public areas with no security controls; shopping centres, public transport, shooting into crowds etc.....
The ability to track terrorists is near impossible. For every lead MI5 and associated networks receive there are probably a dozen individuals and several cells going completely unaccounted for in this country.
These are worrying times