Operation Bertram: Hiding an Army in the Desert

By October 1942 the Eighth Army was going to attack at El Alamein, and Rommel knew it. The line ran from the sea to the Qattara Depression with no flank to turn, so surprise about whether was impossible. What remained available was surprise about where and when.

Operation Bertram, developed under Montgomery with the camouflage officers Geoffrey Barkas, Tony Ayrton and Brian Robb, delivered both.

The where was managed by substitution rather than by concealment, and this is the technically elegant part.

In the northern sector, where the attack would actually fall, tanks were moved in and immediately covered with sunshields — collapsible frames called Diamond and Brian that made a tank look, from the air, like a lorry. In the southern sector, dummy tanks were erected in the positions the real ones had vacated. From the German reconnaissance photographs, nothing changed: the same number of lorries in the north, the same number of tanks in the south, before and after.

This exploits change blindness. Rensink and Simons demonstrated experimentally that observers comparing two versions of a scene detect appearance and disappearance readily but are remarkably poor at detecting substitution — an object being replaced by a different object in the same place. Aerial photographic interpretation is exactly this task, performed at scale under time pressure.

The when was managed with an unfinished pipeline.

A dummy water pipeline was constructed running south, with dummy pump houses and storage. It was built at a deliberate, visible, measurable rate, and its rate of progress implied a completion date in early November. German staff work, doing exactly what competent staff work does, calculated when the southern force would be supplied and therefore when the attack could come.

That is an anchor in the sense of chapter 14, installed by letting the enemy do the arithmetic themselves. A figure a person calculates from evidence is held far more firmly than a figure they are told.

The attack came on 23 October, in the north.

The general principle Bertram demonstrates is that a deception which lets the target derive the false conclusion is stronger than one that asserts it. Nothing was claimed. Objects were placed, and the enemy's own analytic process produced the error.