Soviet troops invaded Afghanistan on 24 December 1979, launching a war which lasted a decade. The BBC has spoken to people on both sides about a conflict which marked their lives.
Afghan refugee Somaya recalls her family's precarious existence in neighbouring Iran as a result of the invasion.
She fears that returning home now, after 20 years, would mean having to start again from scratch.
I am 23-years-old and I have lived in Iran for nearly 20 years.
When we first came to Iran, I was four. That was after the Russians invaded Afghanistan. My father told me that the Russians had killed many people, they put many people in prison - basically, our country was occupied by Russians. We were forced to leave.
Somaya is one of five million Afghan refugees from the war
We tried to cross the border into Iran illegally but we were captured in the middle of the night and sent to a refugee camp.
For a long time, we lived in different camps. Our only possession was a tent, which was hot in summer and cold in winter. Finally, an Iranian man helped us to settle in Masshad, in north-east Iran. But still, our lives were difficult.
My father had been an engineer in Afghanistan and we hoped that he would find a nice job here in Iran but we realised that Afghans were not allowed to work here.
My father was forced to work as a labourer on a building site. Then he worked as a shoemaker in a basement and he could only make enough money to pay the rent and feed us once a day.
The way Iranians treated us made us hate being Afghans. I hid my nationality at school so that I could make some friends. It was very difficult for me to see my other friends, who were better off than I was. We all lived in a damp basement.
My only joy in life was my studies. In all of these years, we were scared that we would be thrown out of the country because the threat was always there from the authorities. We lived with these difficulties in the hope that one day we would return.
But now, three years after the new government in Afghanistan, I still do not want to go back. People there do not even have the basic requirements to live. If we returned, we would have to start from scratch and go through everything we went through in Iran, all over again.
Interviewed by BBC World Service.