From Wikipedia

The Battle of the River Plate

(December 13, 1939) was the first major naval battle in World War II. The German pocket battleship (heavy cruiser) Admiral Graf Spee had been commerce raiding since the start of the war in September. It was found and engaged off the estuary of the River Plate off the coast of Argentina and Uruguay in South America by one of the hunting groups set up by the British Admiralty to search for the Graf Spee, comprising three smaller Royal Navy (RN) cruisers: HMS Exeter, HMS Ajax and HMS Achilles, which was part of the RN's New Zealand Division. HMS Cumberland was also part of the hunting group, but she was refitting in the Falklands.

In the ensuing battle, Exeter was severely damaged and forced to retire, while all other ships received moderate damage. Ajax and Achilles then shadowed the Graf Spee which entered the neutral Uruguayan capital Montevideo. After Hans Langsdorff, the captain of the Graf Spee, was told that the limit of his stay could not be extended beyond 72 hours he scuttled his damaged ship - rather than face the overwhelmingly superior force that the British led him to believe had been assembled.

Although the actual engagement between the German and Allied forces could be regarded as a German victory in terms of losses, the following actions resulted in the overall battle being an Allied victory.