I remember flying into Heathrow once (in the 1970's) and they had tanks on the tarmac for some reason. I've seen drawn guns in Brussels and Moscow, and once in Algeria I had a gun aimed at me for taking out a camera when I was out on the tarmac (yes, I put the camera away).
1970's....IRA bombing campaign. Tanks on the tarmac and security forces inside with machine guns.