Power
Most buildings need a working Power Plant. No power, no factories.
War Factories, Shipyards, Airfields, Refineries and most defences need power. If a base has no working Power Plant, they stop and show Low power.
One Power Plant powers the whole base. A second one gives no extra power, but it keeps the lights on if the first is destroyed.
The Barracks, Machine Gun Nest, Reinforced Bunker and Perimeter Wall work without power.