One letter, published four days before an election, became a mystery that lasted 100 years.

On October 25, 1924, Britain's Daily Mail published a letter said to come from Grigory Zinoviev, head of the Communist International, urging British communists to prepare for revolution. The general election followed on October 29, and the Conservatives won a large victory over the first Labour government.

The Soviet government called the letter a forgery, and most historians now agree it was one. Exactly who wrote it, and how much it really changed the result, is still debated.

Why it still matters: it remains a classic case of how a well-timed document can shape an election.

How do you check whether a shocking news story is real?

#TheHistoryDecodedChannel