On 16 March 1939, an incident occurred involving the nationalist merchant raider, Mar Negro, and the HMS Sussex. The Mar Negro was an armed merchantman of the Nationalist Spanish Navy during the Spanish Civil War. On that day, Mar Negro captured the London-registered freighter Stangate off Valencia. HMS Sussex, a London sub-class County-class heavy cruiser of the Royal Navy, was present but unable to prevent this capture source.
Mar Negro's commander acted quickly by dispatching a prize crew to board the British cargo ship Stangate once Mar Negro got close. The auxiliary cruiser had previously allowed other British ships to proceed after inspections, following orders from the Nationalist high command source.
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These incidents were part of the complex naval interactions during the Spanish Civil War, where neutral British ships sometimes found themselves amidst actions taken by both Republican and Nationalist forces.