This was the Allied movement of British, Australian and New Zealand troops from Egypt to Greece in response to the failed Italian invasion and the likelihood of a German invasion, a threat revealed through 'Ultra' decrypts (5 March/April 1941).

The need for such reinforcements had become apparent earlier in the year, and urgency was added to the contingency planning by the southward movement of German troops from February and by the accession of Bulgaria to the Tripartite Pact on 1 March. It was now seen as politically unacceptable for the UK not to support an ally under threat. Greece had defeated the Italian 'Emergenza G' invasion which had started on 28 October 1940, and was therefore the UK’s only effective ally in Europe. Moreover, use of Greek airfields would put the oilfields of Romania, so important to the German war effort, under threat of British attack. General Sir Archibald Wavell, the British commander-in-chief in the Middle East, was informed during January 1941 that support for Greece must take precedence over all operations in the Middle East, and this order was reinforced in February.

It was assessed that, in purely military terms, British and commonwealth aid would make it possible for the Greek army to hold the Germans at the Aliákmon Line, extending from the Monastir gap in Yugoslavia to the Aegean Sea in the area to the west of the Thracian Khersonnesos, while German deployment to Libya would be ineffectual until the summer of 1941. Both of these assessments proved to be wrong: so far as the former was concerned, the British and commonwealth redeployment to Greece weakened British forces in North Africa to the point at which their 'Compass' offensive failed, and so far as the latter was concerned, the German build-up was more rapid than had been expected by the British, allowing the launch of 'Marita' in April rather than the summer.

From 5 March convoys sailed from Alexandria to Piraeus (the port of Athens) and Vólos (in Thessaly) at regular three-day intervals, escorted by British and Australian warships of Admiral Sir Andrew Cunningham’s Mediterranean Fleet. Each convoy was protected against air attack largely by the light anti-aircraft cruisers Calcutta, Carlisle and Coventry, and covered by a battleship or cruiser together with a group of destroyers. Even so, the convoys lost 25 ships (115,026 tons), though fortunately for the British effort all but seven of these losses occurred in port after the embarked troops had been disembarked.

In all, some 60,364 men and considerable quantities of weapons, equipment and supplies (equivalent to almost four divisions) had been carried across the eastern Mediterranean by 2 April in the face of the distinctly hazardous circumstances of Axis air superiority as manifested by bombing attacks from airfields on Italian-held Rhodes and the Dodecanese islands group. The sole major attempt by the Italian navy to interrupt the convoys was 'Gaudo', which was thwarted by the Italian defeat in the Battle of Cape Matapan.

The forces delivered into Greece comprised Brigadier H. V. S. Charrington’s British 1^st Armoured Brigade Group, Major General B. C. Freyberg’s New Zealand 2^nd Division and Major General Sir Iven Mackay’s Australian 6^th Division, followed by Major General J. D. Laverack’s Australian 7^th Division, while the planned movement of the Polish Brigade Group was cancelled.

The passages to the east and west of Crete and in the area to the south-east of this island were covered by patrols of Italian submarines including Anfitrite, Ondina, Beilul, Galatea, Malachite, Smeraldo, Nereide, Ascianghi, Ambre and Dagabur, but none of these achieved any success and, while attempting to attack the GA.8 convoy on 6 March, Anfitrite was sunk to the east of Crete by the destroyer Greyhound. On 30 March Dagabur was sunk by a British cruiser force, and on 31 March Ambergris was sunk by the light anti-aircraft cruiser Bonaventure.

After their arrival in Greece, the British and commonwealth forces were moved to the north, and two infantry and two armoured divisions had been deployed on the Aliákmon Line, to the south-west of Thessaloníki, before the start of the German, Italian and Bulgarian 'Marita' invasion on 6 April. The Greek army in Thrace and Albania did not retire to the Aliákmon Line as had been arranged, however, and the Allied troops were thereby left very vulnerable.

These forces had little effect on the German invasion and the surviving British, commonwealth and Allied forces were evacuated from Greece in 'Demon' on and after 24 April. Some of these units were moved to Crete in 'Scorcher' and were later overwhelmed by the German 'Merkur' airborne and seaborne invasion of that island.