Bombardment

Several units and fortifications attack automatically from range instead of engaging directly. Bombardment fires on a 15-minute cycle and is one-sided: only specific units can return fire against it.

Bombarding units

Artillery: while idle, projects a 30 km strike zone and bombards every enemy or neutral army inside it. Only artillery can return fire against it.
Jet Fighter: bombards enemy armies within 12.5 km, even while moving. Only Anti-Air and Frigates can return fire.
Anti-Air: automatically bombards hostile aircraft within 50 km.
B2 Bomber: bombards hostile land armies within 25 km, even while moving. Only Anti-Air can return fire.

Bombarding fortifications

Artillery Battery: shells enemy armies near the district.
Automatic Turret: bombards the infantry of hostile land armies within range.
Anti-Air System: bombards hostile air armies within range.

Rules of thumb

Bombardment consumes ammunition; an empty unit or fortification stops firing.
The research focus Artillery Range extends the artillery strike zone.
Army stance matters: a Passive army never fires, not even in retaliation.