The regiment's role was to defend the Suez Canal and Port Suez against air raids by Axis bombers. On 23 August 1941 Muirhead was promoted to acting Brigadier to take command of 2 AA Brigade, but on 3 September he was sent by sea to command 4 AA Bde besieged in Tobruk. This formation of heavy and light AA guns and searchlights was an important part of the Tobruk garrison ('The Rats of Tobruk'), who had been besieged since April. Although the bulk of the garrison had been relieved, most of the AA gunners had served in the town from the beginning. The "Official History" records that the AA artillery in Tobruk was 'incessantly in action against attacks of all kinds, from all heights, but especially by dive-bombers', and that Muirhead and his predecessor were 'responsible for the spirited and successful anti-aircraft defence throughout the siege'. Under Muirhead's command in the last two months of the siege, troops of HAA guns took it in turn to move out to the perimeter and take on ground targets with long-range harassing fire to make up for the shortage of medium artillery. Tobruk was relieved in November 1941 during Operation Crusader.