The Battle of River Plate was one of the first naval battles of WWII in 1939 between the Kriegsmarine and Royal Navy. As it included one pocket battleship and three cruisers it was not an especially large sea battle, but during a period of relative military inactivity it was a notable engagement.
The German pocket battleship Graf Spee had been commerce raiding since the outbreak of hostilities. During such commerce raiding, the Spee had sunk a number of merchant ships. As such, the Royal Navy sent a number of its own ships to sink the Spee. Three of these, the cruisers of the Exeter, Achilles and Ajax, intercepted the Graf Spee at the River Plate estuary.
These Royal Navy cruisers and the Spee met at the Battle of River Plate. Graf Spee's captain, Langdsdorff, duly decided to engage the ships, at first being unaware of the exact ships they were engaging. Originally, such ships were mistaken as destroyers. The Spee closed in on the squadron, which allowed the British to begin their own battle plan.
During the battle itself, however, the Spee escaped damaged. The Spee had also hit and damaged the Royal Navy cruisers, with the HMS Exeter heavily damaged but just remaining afloat. Hereafter, the Graf Spee sped away from the Royal Navy cruisers, to the surprise of Captain Parry who remarked, “'To this day I do not know why the Admiral Graf Spee did not dispose of us in the Ajax and the Achilles as soon as she had finished with the Exeter.”
The battle of River Plate became more a pursuit as the Allied cruisers pursued the Graf Spee. The Spee opened fire on these ships, although the pursuit continued towards the Uruguay coast. The Spee entered the River Plate estuary, and the Achilles again moved to within firing range. However, as it reached the port at Montevideo, the Spee found anchor and the pursuit ended. Royal Navy cruisers remained out of port.
Overall, Uruguay and Montevideo were neutral territory. Consequently, the government of Uruguay stuck rigidly to international law, and the Hague Convention. To quote: "...belligerent war-ships are not permitted to remain in the ports, roadsteads, or territorial waters of the said Power for more than twenty-four hours..."
The writing was on the wall for the Spee as Britain's navy quickly built up reinforcements around the port. Such reinforcements made it increasingly unlikely for the ship to make a clean exit from Montevideo. So when the time came, the Spee began to exit Montevideo and was then scuttled by Langdsdorff.
The Graf Spee was all but destroyed. Not so much by Britain's cruisers during or after the battle in the pursuit to Montevideo, but as a result of the Hague Convention that left the surrounded Spee to either leave port or be interned by the Uruguayans.
With the demise of the Spee, Britain celebrated a victory. The U.K's own warships had remained afloat, although some did need repairs. It was a first naval victory for the Royal Navy surface fleet, and a few more would follow in the years after 1939.
The Phony War was by no means over, but at sea the war was expanding as German U-boat action also increased. British merchant shipping losses were not about to decline and steadily increased as Wolf Packs closed in. Hundreds, even thousands, of British merchant ships were lost in 1940.
The Kriegsmarine did not abandon surface fleet action after the loss of the Spee. The launch of the Bismarck revitalized the German surface fleet in 1940. At 50,000 tons the Bismarck was the largest battleship of the Kriegsmarine, and eclipsed any warship in the Britain's navy.
In 1941, the Reich's navy drafted Operation Rheinübung. This was essentially a new commerce raiding mission which would involve the Bismarck. The primary targets for the battleship were British merchant ships.
However, Scandinavian countries such as Sweden detected the Bismarck, and the Admiralty was informed of German warships. Aerial reconnaissance detected the Bismarck, and then the Suffolk pursued. The Admiralty noted its position in the Kattegat and sent warships to intercept.
At the Battle of Denmark Strait, the Prince of Wales and the Hood engaged the Bismarck. However, they were ineffective and the Hood was lost in battle. After this battle, Britain sent aircraft carrier reinforcements to sink the Bismarck.
Despite this victory, days later British torpedo-bombers bombed the Bismarck and jammed its rudder. This gave the Royal Navy the advantage, and the battleship came under further heavy fire from British warships. Torpedoes ensured the sinking of the Bismarck. Scuttling charges were also left by the crew, which may have also sunk the battleship.
The loss of the Bismarck, after the loss of the Graf Spee, marked the end of the German surface fleet battles with the Royal Navy which had begun at the Battle of River Plate. British naval superiority had ensured the loss of both battleships, so the Third Reich abandoned surface fleet operations. Consequently, U-boats would dominate German naval strategy.