Documents released by the National Archives show 4,000 Britons travelled to Spain in the 1930s to fight General Franco - and that their activities were of great interest to the UK security services.
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Seventy-five years ago they fought - and often died - for a cause in which they believed: combatting fascism in a foreign country.
The Spanish Civil War broke out in the 1930s when an elected left-wing government was overthrown by the army of General Franco.
A stream of idealistically minded, young British men and women volunteered as soldiers in the International Brigades, formed in an unsuccessful attempt to defend the Spanish republic.
But the British Government was neutral, and regarded some of the volunteers as dangerous agitators. Today, for the first time, previously classified intelligence files have been placed online by the National Archives.
They show that 4,000 Britons travelled to Spain - and that then, as now, the movement of UK subjects who go to fight in foreign wars was of great interest to the security services.