The attack that the UK has been preparing for has occurred, a policeman and two members of the public are dead, and more are seriously injured.
Any deaths in a terror attack are tragic, and no doubt there will be urgent investigations into whether PC Keith Palmer could have been better protected, and whether anything could have been done to stop the car driving through a crowd of pedestrians.
And yet the attack has actually demonstrated the success of the exhaustive counter-terror planning and preparation that has taken place in London and across the UK in the last few years.
These low tech, high impact attacks - one man, a car and a knife - are very hard to anticipate and prevent. They require no bomb-making skills or acquiring of firearms, and thus are able to be attempted by any dedicated extremist - today a Frenchman has also been arrested in Belgium on suspicion of driving at a crowd.
Yet the resort to such tactics, by those in the UK trying to carry-out attacks in ISIS or al Qaeda's name, is also a reflection of the successful security and resilience system put in place across Britain, and especially in London.
It is increasingly difficult to access firearms or the chemicals/materials needed to make improvised bombs, and the vast majority of cells that have formed to execute mass casualty attacks have been intercepted over the last decade, resulting more and more in individuals choosing to go it alone.
Additionally, UK efforts to create an 'All Hazards' security and resilience system, designed to make any kind of attack harder to carry-out and have less impact if one does occur, has been very effective. Tragic as the human cost of this attack was, there should be no doubt that without these measures in place the casualty list would have been a lot higher.
There are of course no grounds for complacency, the system can always improve, and lessons will be learned from this attack. But national and London authorities can also move forward with the knowledge that their efforts are making a difference, are working more often than not, and that they have the admiration and support of the British public as they strive to stop similar attacks occurring again.