'Bastion' was a British deception plan created under the supervision of Lieutenant Colonel Dudley W. Clarke’s 'A' Force Middle Eastern group to persuade Generaloberst Erwin Rommel, commander of the Panzerarmee 'Afrika', that his 'Theseus' (i) advance was moving into a carefully conceived British trap as it approached Gazala (1 February/March 1942).

Clarke flew into an airfield near the headquarters of General N. M. Ritchie’s 8^th Army at Gazala in time to see a thick stream of vehicles flowing to the east along the coastal road, and found that the headquarters was packing in preparation for a move back to the Egyptian frontier. By this time Rommel’s forces had advanced some 200 miles (320 km) in a mere 10 days, and the 8^th Army had just two divisions with which to hold a front of 75 miles (120 km). The army required a few days' to ready a defensive line covering Tobruk and to move forward additional divisions from the Nile river delta area.

Clarke immediately drafted the 'Bastion' decoy plan, and in the following three days reconnoitred the terrain and asked for the rapid delivery of other members of his team. On February 5 the plan was complete and the commanders of the 8^th Army and the local element of the RAF commanders agreed to it.

'Bastion' was designed to induce Rommel to pause until the first week in March at the earliest. To achieve this end, the plan proposed to make the German commander believe that he was being led into a trap. The plan’s 'story' was that the British were waiting for Rommel to overcommit himself against the defensive line at Gazala, at which time major armoured reinforcements on the British right in the Tobruk area and the British left deep in the Western Desert would strike to cut him off from his lines of communication. Also included in the plan was the suggestion that Tobruk was to be the advanced base for a British counter-offensive. Clarke worked on the basis that the 'story' would reach Rommel’s staff on 15 February, approach its maximum effect on 20 February, and begin to decline in effect during the first week of March.

The plan’s essential elements were the creation of false armoured reserves on each flank, a simulated strengthening of the Gazala line, and 'evidence' that the British were in Tobruk to stay.

Clarke gave instructions that every dummy tank which could be found was to be moved into Tobruk, and that a force of 300 tanks was to be simulated. Given the fact that the Middle Eastern theatre had nowhere near that required number of dummy tanks, Clarke borrowed a German trick: the RAF had learned to watch Arab encampments for tank tracks, for the Germans had developed the practice of hiding their tanks under Bedouin tents, and Clarke believed that German pilots would also watch for a British adoption of the same subterfuge. The base area at Cairo was able to furnish, in a short time, a supply of unserviceable canvas which could be used to simulate Arab tenting, and during the night of 15/16 February, some 150 such 'tents' were erected deep in the desert behind the left wing of the 8^th Army: four of them featured dummy tanks just visible as through they had been 'hidden' carelessly, the tent area was surrounded by 'tank tracks' created by a special device, and the subterfuge was boosted by campfires and other signs of humanity together with fake radio traffic. Supply convoys were routed past this 'camp' as if dropping off food, ammunition and the like for the tank crews hidden there, and aerial reconnaissance was increased over the southern flank as if in preparation for an attack.

After a few days and several German air attacks, the 'camp' was changed so as to appear abandoned, and the dummy tanks were moved to a nearby wadi.

Meanwhile, 52 dummy tanks were sited at a former airfield in Tobruk itself, where they too were overflown by German pilots, and another 52 were used to 'stiffen' a front-line position. Still more dummy tanks were used to simulate the strengthening of the British main defence line, which also received fake minefields, guns and trenches prepared by camouflage units. Steps were taken to counter the widespread feeling in Tobruk that, unlike the situation during the last German advance when the port had survived an eight-month siege, on this occasion it would be abandoned.

Stories were also planted through the established channels of dissimulation in Portugal and Turkey, and a false story was neatly 'leaked' to a party of disgruntled Vichy French diplomats as they were being repatriated from Cairo to France through Turkey.

Rommel did in fact bring his advance to a halt before the 8^th Army’s Gazala line, and did not resume his offensive until a time late in May.

'Bastion' was also important in helping the 'A' Force to refine its theory and practices, and to bring its art of deception to a higher degree of persuasion.