The 'Battle of Thermopylai' was fought between German and Antipodean troops in Greece within the context of the Germans' 'Marita' invasion of Greece (24/25 April 1941).

After the retreat of the Allied forces from the mountain passes at Olympus and Servia, British commonwealth troops began to establish defensive positions at the coastal pass at Thermopylai, famous for the Battle of Thermopylai in 480 BC. The New Zealand army’s 2^nd Division, under the command of Major-General B. C. Freyberg, was given the task of defending the pass, while elements of the Australian 6^th Division, under the command of Major General I. G. Mackay, defended the nearby village of Brallos.

In the New Zealand sector, the 5^th Brigade was deployed in the foothills along the coast road to the south of Lamia near the Spercheios river, covering the bridges on the road to Larisa. The 4^th Brigade was positioned on the right flank, where it had established coast-watching patrols. The 6^th Brigade was in reserve. The Australian force defending Brallos comprised largely two battalions of the 19^th Brigade, under the command of Brigadier G. A. Vasey: the 2/4^th Battalion and the 2/8^th battalion, and on 19 April these units were augmented by the 2/1^st Battalion and the 2/5^th battalion, from the 16^th Brigade and the 17^th Brigade respectively, which were also put under Vasey’s command. Later that day and during the early hours of 20 April, the 2/11^th Battalion, which was the third battalion of the 19^th Brigade, also arrived at Brallos.

Freyberg and Mackay had been informing their subordinates that there were to be no more withdrawals, but at this time both were unaware of higher-level discussions on the 'Demon' evacuation of the British and allied forces from Greece. After the battle, Mackay was quoted as saying that 'I thought that we’d hang on for about a fortnight and be beaten by weight of numbers.'

On 21 April, German air reconnaissance reported that the defence lines in the Thermopylai area included only light field fortifications that were currently in the initial stage of construction, and that British troops were being evacuated from Salamis island.

On the morning of 23 April, the Australian and New Zealand troops of the ANZAC Corps were ordered to retreat, though it was decided that Thermopylai and Brallos would nonetheless be held by a rearguard comprising two brigades. The New Zealand 6^th Brigade, under the command Brigadier H. E. Barrowclough and the Australian 19^th Brigade were to hold the passes for a time as long as possible in order to make it possible for the other units to withdraw. Vasey said that 'Here we bloody well are and here we bloody well stay.' This was interpreted by Vasey’s brigade major, A. T. J. Bell, as meaning that the brigade would 'hold its present defensive positions come what may', until the withdrawal had been completed.

A German battle group based on the 6^th Gebirgsdivision, under the command of Generalmajor Ferdinand Schörner, attacked at 11.30 on 24 April, meeting fierce resistance. Another battle group, drawn from Generalmajor Gustav Fehn’s 5^th Panzerdivision, also attacked. Early in the afternoon, the German armoured force was shelled near Lamia by artillery that was able to fire down into the pass from several lightly fortified positions. Two of the Panzer division’s tanks later attempted to penetrate the lines of the 25^th Battalion, but were destroyed by long-range field guns. Later in the same day, four German tanks and a number of truckborne infantrymen attempted to take a position in front of the 6^th Brigade ahead of the 25^th Battalion’s position, and after some initial confusion was repelled by 2-pdr anti-tank and 25-pdr field gun/howitzers.

With the delaying action accomplished, the rearguard began to destroy what equipment and field guns they could not transport, and retreated toward another defensive position at Thebes. The last Allied vehicles had been evacuated by 00.00, but Thermopylai was not taken for another three hours, when the Germans launched a frontal attack on the vacated position. While holding their position for the specified amount of time, the Allied forces had destroyed 15 or 16 German tanks and inflicted considerable casualties.

The German forces then continued their march south through Greece, which they then occupied.

The absence of the Greek army from a battle at a site as significant to the national mind-set as Thermopylai, was controversial within Greece, as Antistratigos Georgios Tsolakoglou, commander of the Epiros Army, had already capitulated. After the war, it was argued by some Greeks that this fact was an eternal shame for the Greek régime that did not participate in the battle.