When you get to the combat phase all units from opposing sides in the same sea zone or on the same island
will engage in combat. The name of the phase will tell you two things. The eligible casualties -- air in the air phase, naval in the naval phase, land in the land phase and the amount of dice you will roll.
Each unit that has a value for the phase you are in will add dice to the combat in that phase. This is given as the Air Attack value and so on in the unit description. (Battleships have a value in every phase while infantry only in land attack.)
In a sea zone with naval units and both fighters and bombers during the attack air units phase you will add the air attack dice
up from both the naval and air units in the zone. Shake up the battle box and pull out the neck to the counted number of dice and read the casualties from the air line. Then do so for the other side. Combat is considered simultaneous. Remove the destroyed units from each side.
Proceed to the next zone with air units. When all air phase combat is complete move on to the next phase.
In the naval phase any planes that survived the enemy planes and AA fire from ships or AA guns on islands will now get to contribute dice to the attack sea units
Note that artillery may fire into one adjacent zone in a naval attack phase and ships may fire onto one adjacent island in the attack land units
phase. Combat does not continue until one side retreats or is eliminated.
Both sides may still have units in the same sea zone or on the same island at the end of all combat phases. RulebookFAQ