Operation Substance and layered Malta convoy defense

Operation Substance resupplied Malta through a Mediterranean route chosen to manage shallow water and concentrations of enemy aircraft. Sicily's mountainous terrain complicated airfield supply, while North Africa offered more infrastructure for enemy aircraft, shaping where the convoy expected air attack. Heavy covering units accompanied the convoy partway, while Edinburgh and the close escort of Force X continued through the narrows. The formation placed strong cruisers and Tribal-class destroyers toward the northern side, where an Italian surface attack was expected. Air attacks remained the most immediate danger, but the escort was organized to answer several threat types at once. The convoy's security depended on layered forces around and beyond the merchant formation.