Tanks at Heathrow Airport
On February 17th 2003, there was a 'terror alert' in the UK and the United States. In the UK, various security measures were increased, and there was strengthened police prescence at various sites over London.
Also, at Heathrow Airport, the largest airport in the UK, and one of the largest hubs in the world '450 troops have joined 1,000 extra police officers in patrolling the airport' 
"The prime minister's spokesman said action to deploy troops at Heathrow would not have been taken unless it was thought to be necessary."
"The move was "an ongoing operation in relation to a specific threat", the spokesman said."
The oddest part was were the "armoured vehicles within Heathrow." - It was wondered exactly what they were going to do.
The prime minister, Tony Blair, personally authorised the use of troops as a precautionary action linked to fears that al-Qaida could use the end of the Muslim festival of Eid al-Adha on Sunday to mount attacks, according to Mr. Blair's official spokesman.
However, the week before, on Tuesday 11th February 2003, tanks and troops had already been deployed to Heathrow in response to another 'terror alert'. 
This was four days before the planned anti-war march to protest the imminent invasion of Iraq, the biggest anti-war demo in history (22 million worldwide).
Many suspected that the deployment of tanks was a PR stunt to engender fear in the population and persude them that the imminent war was legitimate.
It was later reported that they were a "public relations disaster" for the UK tourism industry.