The square mile around Westminster in London is one of the most heavily policed areas in the world.
A high security presence is always visible and additional precautions have been taken in the post 9/11 era of modern terrorism.
And yet it is virtually impossible to secure anywhere.
Scenes of panic and fear returned to the streets of London yesterday when a man drove an SUV across Westminster Bridge, ploughing into innocent pedestrians.
After ramming the railings of parliament and attacking a police officer, the assailant was shot.
The motivation of the attacker is not yet known.
But yesterday's horror has joined the growing list of terror incidents of our times.
The attack in Nice last summer showed how horrifically easy it is for a terrorist driving a vehicle to wreak murder and mayhem. London saw this tactic in action in broad daylight yesterday when a 4x4 caused such destruction.
The swift response of the police and emergency services in London deserves to be commended. Passers-by came to the aid of those who were injured.
The deaths and catastrophic injuries caused in Westminster Bridge and parliament yesterday are, unfortunately, a stark reminder of the necessity to be vigilant.
The latest counter-terror clampdown sees British authorities banning people flying with large electronic devices from certain predominantly Muslim countries.
Officials claim the decisions come after specific intelligence reports suggesting electronic devices could be used for an attack if they were carried into a plane's cabin.
The move was scoffed at in recent days. But now, further anti-terrorism measures will be expected.
Despite these measures, the world feels less safe.