Tensions have escalated following the expulsion of 60 Russian diplomats from the US
It was a dramatic show of solidarity that has escalated long-standing tensions between Russia and the West.
On Monday, the US and its allies expelled dozens of Russian diplomats in response to the poisoning of a former Russian spy in the UK.
It was the biggest such expulsion since the height of the Cold War era and the hostilities with the then Soviet Union.
There are now fears of a serious diplomatic crisis and a freezing in relations between Moscow and the West which has raised the spectre of the Cold War once again.
Dozens of Russian diplomats expelled
What the diplomat expulsions tell us
Full coverage of Russia spy poisoning
What was the Cold War?
This term describes the tense relationship between the US and the Soviet Union from 1945 to 1989.
Neither side ever fought the other directly because the fear of nuclear war, which gripped millions of people at the time, was too terrifying to contemplate.
Instead, historians consider it to be a war between two opposing systems of government. The US and the West represented capitalism and the Soviet Union represented communism.
Both sides had very different ideas of how to run a country and both believed their system was superior. A source of major tension was that they both believed the other side was trying to spread their belief system around the world.
Image copyrightNASA
Image caption
US astronaut Neil Armstrong on the moon. Space exploration was the focus of fierce competition during the Cold War
How did it begin?
There is no single answer to this, but historians generally point to the end of World War Two in 1945 as a key turning point.
This is because during the war the US and Soviet Union had been allies, but the relationship was forged in the face of a common enemy in Nazi Germany and did not last.
The war left Europe divided and the two sides emerged as the world's most dominant superpowers.
Given their opposing belief systems, there were disagreements about how the post-war world should be ordered and how Europe should be divided up.
This sparked a fierce rivalry and a freezing in relations as both powers competed for dominance.
