Practice Relating to Rule 54. Attacks against Objects Indispensable to the Survival of the Civilian Population
Section B. Attacks against objects used to sustain or support the adverse party
Sweden’s IHL Manual (1991) states:
It is permitted to attack stocks of foodstuffs, water reservoirs, etc. which are in the hands of the adversary’s armed forces. In practice, however, it would probably be very hard to determine whether a food transport or store was intended only for the armed forces or also for the civilian population. Also, military food transports may in some cases be intended for protected groups such as prisoners-of-war or civilians in the hands of one of the belligerents.
… Attacks may also be made on objects being used by the adverse party in direct support of his military operations. This exception may apply mainly when enemy units are for example using a cornfield for advance, or some other object for concealing military units.
The text uses the expression “military action” as opposed to the often-used expression “military operations” which is a broader concept. Thus the exception applies only if the attack entails a direct
advantage in a given tactical situation. As against this, it is not permitted to attack an irrigation works, for example, with the excuse that this may be an advantage in a future operation, i.e. an indirect advantage.
[emphasis in original]