The 'Battle of the River Plate' was fought in the South Atlantic as the first naval battle of World War II, and involved the German 'pocket battleship' (Panzerschiff in German) Admiral Graf Spee, commanded by Kapitän Hans Langsdorff, and a Royal Navy squadron commanded by Commodore Henry Harwood and comprising the heavy cruiser Exeter and the light cruisers Ajax and New Zealand Achilles (13 December 1939).

Supported by the supply and tanker vessel Altmark, Admiral Graf Spee had sailed from Wilhelmshaven for the South Atlantic on 8 August, before the start of the war, and embarked on a commerce-raiding foray in the South Atlantic and Indian Oceans after receiving authorisation on 26 September. Armed with six 283-mm (11.14-in) main guns in two turrets, the pocket battleship sank several merchantmen without loss of life as Langsdorff operated a policy of taking all crews on board before sinking their ships.

The Royal Navy assembled nine forces to search for the surface raider: Force 'G', the South American Cruiser Squadron, comprised the heavy cruiser Cumberland armed with eight 8-in (203.2-mm) guns in four turrets, the heavy cruiser Exeter with six 8-in (203.2-mm) guns in three turrets, and two light cruisers in the form of Ajax and Achilles with eight 6-in (152.4-mm) guns in four turrets. In the period before and during the battle, Cumberland was refitting in the Falkland Islands but was available for sea at short notice. Force 'G' was supported by the oilers Olna, Olynthus and Orangeleaf. Olynthus replenished Ajax and Achilles on 22 November and Exeter on 26 November in San Borombon Bay. Olynthus was also directed to keep watch between Medanos and Cape San Antonio, off Argentina to the south of the Plata river estuary.

Following a raider-warning radio message from the merchantman Doric Star, which was sunk by Admiral Graf Spee off South Africa, Harwood suspected that the raider would try to strike next at the merchant shipping off the Plata river estuary between Uruguay and Argentina, and therefore ordered his squadron to steam toward the position 32° S and 47° W. Harwood chose this position as it was the most congested part of the shipping lanes in the South Atlantic, and therefore the point where a raider could find and do the greatest damage to Allied shipping. A Norwegian freighter spotted the German warship practising the use of her searchlights and radioed that her course was toward South America. The three British cruisers made rendezvoused off the estuary on 12 December and undertook manoeuvres.

The British combat instructions for the engagement of a 'pocket battleship' by a cruiser squadron had been devised by Harwood himself during his period at the Royal Naval War College between 1934 and 1936. The tactical instructions specified an immediate attack by day or night. If during the day, the ships would attack as two units, in this case with Exeter separate from Ajax and Achilles, and if at night the ships would remain in company, but in open order. By attacking from two sides, Harwood hoped to give his lighter warships a chance of overcoming the German advantage of greater range and heavier broadside by dividing the fire of the 'pocket battleship'. By dividing his force, Harwood would force the Germans to either split their fire, reducing its effectiveness, or keep it focused on one opponent, allowing the other vessels to attack with less fear of return fire.

Although outgunned by Admiral Graf Spee and therefore at a tactical disadvantage, the British did have the strategic upper hand as any raider returning to Germany would have to run the final gauntlet of the North Sea and might reasonably be expected to be met by the heavy warships of Admiral Sir Charles Forbes’s Home Fleet. For victory, the British only had to damage the raider enough that she was either unable to make the journey or unable to fight a subsequent battle with the Home Fleet. Because of the UK’s overwhelming numerical superiority in warships, the loss of even as many as three cruisers would not have severely diminished the UK’s naval capabilities, whereas Admiral Graf Spee was one of the German navy’s very few capital ships. The British could therefore afford to risk a tactical defeat if paved the way to a strategic victory.

At 05.20 on 13 December, the British squadron was proceeding on a course of 060° at 14 kt some 450 miles (720 km) to the east of Montevideo, the capital of Uruguay on the northern coast of the Plata river estuary. At 06.10, smoke was sighted to the north-west, and Harwood ordered Exeter to investigate. The heavy cruiser swung out of line and at 06.16 signalled by lamp that 'I think it is a pocket battleship'. Captain F. S. Bell ordered Flag N (Enemy in Sight) hoisted to the yard arm. Admiral Graf Spee had already sighted mastheads and identified Exeter, but initially suspected that the two smaller warships were not light cruisers but less threatening destroyers, and that the British ships were protecting a merchant convoy whose destruction would be a major success. As Admiral Graf Spee's Heinkel He 60 single-engined reconnaissance floatplane was currently unserviceable, Langsdorff relied on his look-outs for this information. He decided to engage, despite the fact that he had received a broadly accurate report from the German naval staff on 4 December, outlining British activity in area of the Plata river estuary. This report included information that Ajax, Achilles, Exeter and Cumberland were patrolling the eastern seaboard of South America.

Langsdorff realised too late that he was facing three cruisers. Calling on the immediate acceleration of his ship’s Diesel engines, he closed with the British squadron at 24 kt in the hope of engaging the steam-powered British ships before they could work up from cruising to full speed. This tactic was probably an inexplicable blunder: Langsdorff could perhaps have manoeuvred to keep the British ships at a range where he could destroy them with his 283-mm (11.14-in) guns while remaining out of the effective range of the latter’s 8- and 6-in (203.2- and 152.4-mm) guns. On the other hand, Langsdorff knew that the British cruisers had a speed advantage over his ship of between 4 and 6 kt, and could in theory remain out of the German guns' range should they choose to do so: this was the standard cruiser tactic in the presence of a superior force while calling for reinforcement.

The British executed their battle plan: Exeter turned to the north-west, while Ajax and Achilles, operating together, turned to the north-east to divide Admiral Graf Spee's fire. The German ship opened fire on Exeter at a range of 18,590 yards (17000 m) at 06.18. Exeter's forward guns responded at 06.20 and those of Achilles at 06.21, while Exeter's aft guns opened fire at 06.22 and those of Ajax at 06.23.

From the first salvo, Admiral Graf Spee fire proved fairly accurate, her third salvo straddling Exeter. At 06.23, a 283-mm (11.14-in) shell burst just short of but abreast Exeter, and the shell’s splinters killed the torpedo-tube crews, damaged the ship’s communications, riddled the ship’s funnels and searchlights, and wrecked the ship’s Supermarine Walrus single-engined flying boat just as it was about to be catapult-launched for gunnery spotting. Three minutes later, Exeter took a direct hit on her 'B' turret, putting it and its two guns out of action. Fragments swept the bridge, killing or wounding all of the bridge personnel except the captain and two others. Bell’s communications were wrecked, communications from the aft conning position were also destroyed, and the ship had to be steered via a chain of messengers for the rest of the battle.

Meanwhile, Ajax and Achilles had closed to a range of 13,000 yards (11890 m) and started to overhaul the German ship, causing her to divide her main armament at 06.30 and otherwise use her 149.1-mm (5.87-in) secondary armament against them. Shortly after this, Exeter fired two torpedoes from her starboard tubes, but both missed. At 06.37, Ajax catapult-launched her Fairey Seafox single-engined spotter floatplane. At 06.38, Exeter turned so that she could fire her port torpedoes, and received two more direct hits from 283-mm (11.14-in) shells: one impacted 'A' turret and put it out of action, and the other entered the hull and started fires. At this point, Exeter was severely damaged, having only 'Y' turret still in action under local control with an officer on its roof shouting instructions to those inside. The heavy cruiser also had a 7° list, was being flooded and being steered with the use of her small boat’s compass. However, it was Exeter which dealt the decisive blow: one of her 8-in (203.2-mm) shells had penetrated two decks before exploding in the area of Admiral Graf Spee's funnel, destroying her raw fuel processing system and leaving her with fuel for just 16 hours, wholly insufficient to allow the ship to return to Germany.

At this point, nearly one hour after the start of the battle, Admiral Graf Spee was doomed as her crew could not make fuel system repairs of this complexity under fire. Two-thirds of her anti-aircraft guns (six 105-mm [4.13-in], four 37-mm and 10 20-mm weapons) had been knocked out, as well as one of her 149.1-mm (5.87-in) secondary turrets. There were no friendly naval bases within reach, nor was any reinforcement available. The ship was no longer seaworthy and her only option was therefore to make it into neutral port of Montevideo.

Admiral Graf Spee then turned from an easterly course, now behind Ajax and Achilles, toward the north-west and laid smoke. This course brought Langsdorff roughly parallel to Exeter which, by 06.50 was listing heavily to starboard and taking water forward. Nevertheless, she still steamed at full speed and maintained fire with her one remaining turret. Forty minutes later, water splash from the near-miss of a 283-mm (11.14-in) shell short-circuited her electrical system for that turret. Bell was thereby compelled to break off the action. This was the opportunity for Admiral Graf Spee to sink Exeter, but instead the combined fire of Ajax and Achilles drew Langsdorff’s attention as both of the British light cruisers closed the German ship.

Twenty minutes later, Ajax and Achilles turned to starboard to bring all their guns to bear, causing Admiral Graf Spee to turn away and lay a smoke screen. At 07.10, the two light cruisers turned to reduce the range from 12,500 yards (11430 m), even though this meant that only their forward guns could fire. At 07.16, Admiral Graf Spee turned to port and headed straight for the badly damaged Exeter, but at 07.20 the fire from Ajax and Achilles forced her to turn and engage them with her main armament, while they turned to starboard to bring all their guns to bear. Ajax turned at 07.24 and fired her torpedoes at a range of 7,000 yards (6400 m), causing Admiral Graf Spee to turn away under cover of a smokescreen. At 07.25, Ajax was hit by a 283-mm (11.14-in) shell that put 'X' turret out of action, jammed 'Y' turret and caused number of casualties. By 07.40, Ajax and Achilles were running low on resources, and the British decided to change tactics, moving to the east under cover of a smokescreen. Harwood decided to shadow Admiral Graf Spee and try to attack at night, when he could employ torpedoes and make better use of his advantages in speed and manoeuvrability while minimising his deficiencies in armour protection. Ajax was again hit by a 283-mm (11.14-in) shell whose detonation destroyed her mast and caused more casualties, and Admiral Graf Spee continued to the south-west.

The battle now turned into a pursuit. The British and New Zealand light cruisers separated, keeping about 23,500 yards (21500 m) from Admiral Graf Spee with Ajax and Achilles respectively to port and starboard of the German ship. At 09.15, Ajax recovered her spotter seaplane. At 09.46, Harwood signalled to Cumberland for reinforcement, and the Admiralty also ordered ships within 3,000 miles (4830 km) to proceed to the Plata river area. At 10.05, Achilles realised that she had overestimated Admiral Graf Spee's speed as she now came within range of the German ship’s guns. Admiral Graf Spee turned and fired two three-gun salvoes with her forward guns, and Achilles turned away under smoke.

At 11.03 a merchant ship was sighted close to Admiral Graf Spee. and a few minutes later the German ship radioed Ajax, on the international watch-keeping and using each ship’s pre-war call sign, with a signal requesting that the light cruiser pick up the lifeboats of an 'English steamer'. The German call sign was DTGS, confirming to Harwood that the pocket battleship was indeed Admiral Graf Spee rather than her sister ship Admiral Scheer. Ajax made no reply, but a little later the British ship, which was Harwood’s flagship, closed with Shakespeare, which still had her lifeboats still hoisted and men still on board. Admiral Graf Spee had fired a gun and ordered this British merchant vessel to stop, but when this produced no result, Langsdorff had decided to continue on his way and Shakespeare therefore had a lucky escape. The shadowing continued for the rest of the day until 19.15, when Admiral Graf Spee turned and opened fire on Ajax, which turned away under smoke.

It was now evident that Admiral Graf Spee was entering the Plata river estuary. Since this was characterised by a number of sandbanks, Harwood ordered Achilles to shadow the German ship while Ajax covered any attempt by the German ship to double back through a different channel. The sun set at 20.48, with Admiral Graf Spee silhouetted against the sun. Achilles had again closed the range and Admiral Graf Spee opened fire, again forcing Achilles to turn away.

During the battle, 108 men had been killed on both sides, the total including 36 men on Admiral Graf Spee.

Admiral Graf Spee entered the harbour of Montevideo in neutral Uruguay, dropping anchor at about 00.10 on 14 December. This was a political error as Uruguay, though neutral, had benefitted from significant British influence during its development and favoured the Allies. The port of Mar del Plata on the Argentine coast some 200 miles (320 km) to the south of Montevideo, would have been a better choice. Also, had Admiral Graf Spee left port at this time, the damaged Ajax and Achilles would have been the only British warships which it would encounter in the area.

In Montevideo, the terms of the 13^th Hague Convention came into play. Under Article 12, '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', and under Article 14, a 'belligerent war-ship may not prolong its stay in a neutral port beyond the permissible time except on account of damage'. British diplomats duly pressed for the speedy departure of Admiral Graf Spee. Also relevant was Article 16, part of which reads 'A belligerent war-ship may not leave a neutral port or roadstead until twenty-four hours after the departure of a merchant ship flying the flag of its adversary'.

In accordance with their obligations, the Germans now released 61 captive British merchant seamen who had been on board. Langsdorff then asked the Uruguayan government for two weeks to make repairs. Initially, British diplomats in Uruguay made several requests for Admiral Graf Spee to leave port immediately, but after consultation with London, which was aware that there were no significant British naval forces in the area, continued to demand the departure of Admiral Graf Spee. At the same time, the British ambassador arranged for British and French merchant ships to steam from Montevideo at intervals of 24 hours, whether they had originally intended to do so or not, thus triggering the terms of Article 16. This kept Admiral Graf Spee in port and thus provided more time for British forces to gather the area.

At the same time, the British attempted to feed false intelligence to the Germans to there effect that an overwhelmingly strong British force was being assembled. The British intimated though a number of broadcast signals, on frequencies known to be monitored by the Germans, that this included the fleet carrier Ark Royal and the battle-cruiser Renown. In fact the two light cruisers had been joined only by Cumberland, which had arrived at 22.00 on 14 December, after steaming 1,167 miles (1878 km) from the Falkland Islands in 34 hours at an average of more than 90% of her full trials speed attained over much shorter distances. The older and larger Cumberland was more powerful than Exeter, with an additional after turret containing two more 8-in (203.2-mm) guns, but was no match on paper for Admiral Graf Spee, whose guns had significantly longer range and fired 300-kg (661-lb) shells rather than the British ship’s 256-lb (116-kg) shells. Overwhelming British forces, in the form of Renown, Ark Royal, the heavy cruisers Shropshire and Dorsetshire, and the light cruiser Neptune, were on their way, but would not assemble until 19 December. Even so, these reinforcements could have intercepted earlier had Admiral Graf Spee headed north or north-east from Montevideo shadowed by Cumberland and the two light cruisers. For the time being, the total British force comprised the undamaged Cumberland with a full ammunition load, and the damaged Ajax and Achilles with depleted ammunition loads. To reinforce the propaganda effect, these ships, which were waiting just outside the 3-mile (4.8-km) limit, were instructed to make smoke, which could be clearly seen from the waterfront in Montevideo.

On 15 December, Olynthus refuelled Ajax, a task which proved difficult, and two days later refuelled Achilles.

The Germans were wholly deceived, and expected to face a far superior force on leaving the Plata river estuary. Also factored into the German thinking was the fact that Admiral Graf Spee had expended some two-thirds of her 283-mm (11.14-in) ammunition and had left sufficient for only about 20 minutes of firing. Such a reduced ammunition stock was totally insufficient for the ship even to fight her way out of Montevideo, let alone get back to Germany even if she had the fuel to do so, when contrasted with the previously unengaged Cumberland's ability to fight at full capacity for about 90 minutes and pursue at equal or higher speed for at least 2,300 miles (3700 km) before needing replenishment at sea.

Meanwhile, as Admiral Graf Spee lingered in the bay, British diplomats in Montevideo and Buenos Aires carefully watched from shore, and the expectation of a potential break-out or a resumption of the battle caused tensions and anxiety to surge to enormous levels among British sailors and diplomats.

On the German side, while the ship was prevented from leaving harbour, Langsdorff consulted with his command in Germany. He received orders that permitted some options, but not internment in Uruguay, for the Germans feared that Uruguay could be persuaded to join the Allied cause. Ultimately, Langsdorff opted to scuttle his ship in the Plata river estuary on 17 December, to avoid unnecessary loss of life for no particular military advantage: this decision and its implementation infuriated Adolf Hitler. The crew of the German warship was taken to Buenos Aires in Argentina, where Langsdorff committed suicide on 19 December. It was reported that many of the crew were moved to Montevideo with the help of local people of German origin. The German dead were buried in Montevideo.

German propaganda had already reported that Admiral Graf Spee had sunk one heavy cruiser and severely damaged two light cruisers in exchange for lightly damage to herself. Thus the scuttling of Admiral Graf Spee came as a severe embarrassment that was difficult to explain on the basis of publicly available facts. The battle was a major victory for the British, as the damage to Ajax and Achilles was not sufficient to reduce their fighting capacities, while Exeter, as badly damaged as she was, was able to reach the Falkland Islands for emergency repairs before returning to Devonport for a 13-month refit, thus enhancing the reputation of Winston Churchill, the First Lord of the Admiralty. While being highly praised for his excellent performance in battle, Harwood also received criticism about his lack of initiative and for not employing a more aggressive approach: these criticisms were centred mostly on the fact that Admiral Graf Spee had been allowed to escape even though she was outgunned and outnumbered.

Most of the prisoners taken from merchant ships by Admiral Graf Spee had already been transferred to her supply ship Altmark, and were freed by a boarding party from the British destroyer Cossack in Norwegian neutral waters in the course of the 'Altmark Incident' of 16 February 1940. As noted above, prisoners who had not been transferred to Altmark had remained aboard Admiral Graf Spee during the battle, and were released on arrival in Montevideo.

On 22 December, more than 1,000 of Admiral Graf Spee's men were taken to Buenos Aires for internment. At least 92 of this total were transferred during 1940 to a camp in Rosario, some were transferred to Club Hotel de la Ventana in Buenos Aires province, and another group to Villa General Belgrano, a small town founded by German immigrants in 1932.