On 9th November 1989 the Berlin Wall fell - one in a series of monumental events that went on to transform Europe, and also the work of MI5.
The wall had been put up in August 1961 to stem the massive flow of refugees from East to West Germany after Germany was divided into two separate countries in 1949. This outpouring represented a great loss of prestige for the East German regime, as well as an enormous economic loss: many of those fleeing west were scientists, engineers and doctors. The barbed wire and fences went up along the border overnight on 12 August, meaning people were stuck on whichever side they had gone to bed on that night.
The fall began on the evening of 9 November after mass demonstrations in East Berlin resulted in the East German government announcing that all its borders would be opened. People soon took matters into their own hands and started to break through the wall in other areas with whatever implements they had to hand, before the Mauerspechte - Wall Woodpeckers - went to work on the official demolition, to the cheers of crowds of onlookers.
The collapse of the Soviet bloc and Soviet Union itself led to the end of the Cold War, and a resultant decline in the threat from subversion and espionage. This coincided with a shift in the domestic and global terrorist landscape, meaning MI5's focus shifted increasingly towards counter-terrorism rather than counter-espionage and counter-intelligence. The bombing of Pan-Am flight 103 over Lockerbie in Scotland - still the highest-casualty terrorist attack in the UK - had taken place less than a year before, and the Provisional Irish Republican Army (PIRA) was well-armed thanks to significant shipments of weaponry from Libya during the 1980s. These and other areas of counter-terrorism work, including the ongoing hostage crisis in Lebanon, demanded a significant MI5 investigative response.
The peace dividend arising from the end of the Cold War, and the escalating pace of the threat from Northern-Ireland Related Terrorism, led to an expansion in our remit, with MI5 taking on the national lead for investigating Irish Republican terrorism on the UK mainland for the first time.
In the years that followed, MI5 also played an important role in supporting the development of the fledgling security and intelligence services in the emerging democracies of Eastern Europe.
Today, very little of the wall is left. Just three sections remain standing: a 90m piece at the site of the former Gestapo HQ, halfway between Checkpoint Charlie and Potsdamer Platz, another section along the river Spree near Oberbaun Bridge and a third at a site in Bernauer Strasse, which was turned into a memorial in 1999. Those parts which do remain are a stark reminder of the Cold War years - an incredibly challenging and important period in MI5's history - and the turning point which its destruction marked.