It was seen as politically unacceptable not to support an ally under threat. Greece had defeated the Italian invasion and was therefore Britain's only effective ally in Europe. In addition, use of Greek airfields would put the Romanian oilfields, vital to Germany's war effort, within reach of Allied bombers. General Archibald Wavell, the Allied commander in North Africa, was told in January 1941 that support for Greece must take precedence over all operations in the Middle East and this order was reinforced in February.
Wavell's attitude is unclear. It had been generally believed that he was pushed into the Greek campaign but recent writers believe that Wavell was convinced of it. Militarily, it was assessed that, with help, the Greek Army could hold the Germans at the Aliakmon Line, while German deployment to Libya would be ineffectual until the summer. Both assessments were wrong. Historians now see that this Allied deployment weakened British forces in North Africa to a point where their Operation Compass offensive failed.
From March 6, convoys moved from Alexandria to Piraeus at regular 3 day intervals, escorted by warships of the Royal Navy and the Royal Australian Navy. Although there were air attacks, these had little effect. The major attempt by the Italian Navy to interrupt the convoys was thwarted by the Battle of Cape Matapan. About 58,000 men and their equipment were moved to Greece by 2 April. These comprised the British 1st Armoured Brigade, the New Zealand Division and the 6th Australian Division, followed by the 7th Australian Division and the Polish Brigade.
Two infantry and two armored divisions were in place on the Aliakmon Line, south-west of Thessaloniki (Salonica), before the Axis (German, Italian and Bulgarian) invasion (Operation Marita) on the 6th April. The Greek Army did not retire, however, to the Aliakmon Line as expected and the Allied troops were thereby vulnerable. These forces had little effect on the German invasion and they were evacuated (Operation Demon) on and after 24th April.
Some of these units were moved to Crete (Operation Scorcher), where they were overwhelmed by the airborne invasion of that island (Operation Merkur), although it was a Pyrrhic victory for the Germans.