Infrastructure is a legitimate target in mass warfare. You try to influence the civilian population to pressure the government to end the conflict as well as to destroy the logistical infrastructure that supports the war.
For example...if a civilian airfield has a runway long enough for fighter or bomber aircraft and can be used for lateral dispersal, then it would be a legitimate target.
Same thing applies to electrical infrastructure. If a military base has its own electrical plant, but it is connected to the civilian electrical network as a backup, then the civilian electrical network is a legitimate target.
The list goes on:
Communications towers
Radio and TV towers
Satellite antennae
Bridges
Road Junctions
Railroad stations