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 three smaller Royal Navy (RN) cruisers: HMS Exeter, HMS Ajax and HMS Achilles,She was HMS Achilles at the time of the battle. She became HMNZS Achilles on the formation of the Royal New Zealand Navy in 1941 which was part of the RN"s New Zealand Division.

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 a tense period, the captain of the Graf Spee, Hans Langsdorff, scuttled his damaged ship rather than face the overwhelmingly superior force that the British had led him to believe had 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.