How are ground combat mods figured out when different combinations of units are involved? EX. a ground unit defends vs an amphib assault but has another unit added to defense via amph reaction. do all units get amph modifier. what if a 2 step large unit is supported by a Japan naval Brigade , do they then supply the opposing side the mod for adding a amph unit? any unit that started in hex gets a +3 so if anenemy moved into hex to attack and you react a unit amphibiously it would not get +3, added unit would be half defense?
First off, the +3 AA mod only applies to the reaction player (see 8.4). The reaction units are presumed to have arrived before the invasion, so they do not suffer any adverse modifiers or defense halving as do the offensive units.
When there is a mix of offensive units, some doing amphibious assault and others attacking overland, the reaction player gets the +3 drm, but the defensive halving only applies to the Offensive units that are conducting AA.