HASs are a passive defence measure (i.e., they limit the effect of an attack, as opposed to active defences; e.g., surface-to-air missiles) which aim to prevent or at least degrade enemy attacks. The widespread adoption of hardened aircraft shelters can be traced back to lessons learned in the 1967 Arab-Israeli Six-Day War when the Israeli Air Force decimated the unprotected Egyptian Air Force, at the time the largest and most advanced air force in the Arab world, at its airbases.