The Battle of the River Plate was fought in the South Atlantic on 13 December 1939 as the first naval battle of the Second World War. The Kriegsmarine heavy cruiser Admiral Graf Spee, commanded by Captain Hans Langsdorff, engaged a Royal Navy squadron, commanded by Commodore Henry Harwood, comprising the light cruisers HMS Ajax, HMS Achilles (on loan to the New Zealand Division) and the heavy cruiser HMS Exeter.

Graf Spee had sailed into the South Atlantic in August 1939, before the war began, and had begun commerce raiding after receiving appropriate authorisation on 26 September 1939. Harwood's squadron was one of several search groups sent in pursuit by the British Admiralty. They sighted Graf Spee off the estuary of the River Plate near the coasts of Argentina and Uruguay.

In the ensuing battle, Exeter was severely damaged and forced to retire; Ajax and Achilles suffered moderate damage. Damage to Graf Spee, although not extensive, was critical because her fuel system was crippled. Ajax and Achilles shadowed the German ship until she entered the port of Montevideo, the capital city of neutral Uruguay, to effect urgent repairs. Langsdorff was told that his stay could not be extended beyond 72 hours. Apparently believing that the British had gathered a superior force to await his departure, he ordered the ship to be scuttled. Three days later, Langsdorff killed himself.