The shocking and brutal murder of so many innocent holiday-makers in Sousse has yet again brought the reality of terrorism to the fore.
Today we remember the victims of another dreadful day. We all know where we were that Thursday morning ten years ago when 52 people died at the hands of terrorists in London. I remember all too well an ominous news report about disruption caused by "power surges" on the Tube. The truth about those disgusting murders, the injuries to hundreds of others, and the many lives affected quickly became apparent.
I had taken over as MI5's Counter Terrorism director a few months earlier. We had always known - and said publicly - we simply can't find and stop every terrorist plot. We could not have prevented 7/7. While it remains true that we thwart most attempts, the rare occasions when terrorist attacks occur stand as stark moments in contemporary history.
In the preceding months, there had been a degree of scepticism about the terrorism threat in the media: surely it couldn't happen here? The fact of 7/7 ended those arguments and led to a step change in the nation's counter-terrorism defences. A year later we detected and, with partner agencies, prevented Al Qaida's most ambitious plot - to bring down multiple airliners on US cities using liquid bombs on flights from London. Thousands would have died. I'm not sure we would have detected it without the uplift that followed 7/7.
These and other appalling acts are attempted by individuals who have grown up here but decided for whatever twisted reasons to identify their own country as the enemy. They are a tiny fraction of the population. But the continuing fact that some people, born in the UK, with all the opportunities and freedoms that modern Britain offers, can nonetheless make those sorts of warped choices presents a serious societal and security challenge.
The terrible events in London on 7 July 2005 are enduring reminders of the reality of what MI5 is striving every day to prevent.
- Director General Andrew Parker