Here are the d-fire rules…
Just keep it simple… when you move a unit… from one hex to another… if both those hexes are adjacent to an enemy unit, then that enemy unit can try to disrupt you.
Adjacent means same hex or bordering hex.
In some cases, you are forced to use defensive fire while the moving enemy unit is in a particular hex. For instance, if an enemy tank tries to move out of a hex containing one of your soldiers, if you choose to use defensive fire, you would likely have to do so while the tank is in the first hex (your hex). That’s because once the tank is in the next hex (one hex away), most soldiers attack value at Range 1 can’t result in enough successes to match the defense of the tank to disrupt it.
Provoking Defensive Fire: A unit provokes defensive fire when it moves from one hex adjacent to an enemy unit into another hex adjacent to that same enemy unit. The hex a unit is in counts as adjacent to that unit.
Soldiers and Vehicles: Soldiers don’t provoke defensive fire from Vehicles. They can move around an enemy tank or even enter its hex without getting shot at by that Vehicle. Soldiers can exploit enemy Vehicles’ blind spots to move safely around them, but enemy Soldiers see them just fine.
Defensive-Fire Limit: A given unit can make only one defensive-fire attack per phase. For example, a US bazooka team fires defensively against a German tank in the Axis player’s movement phase, but when a second German tank moves into the bazooka team’s hex in the same movement phase, the bazooka team can’t use defensive fire—it’s already fired defensively this phase. During the Axis player’s assault phase, the bazooka team will be able to use defensive fire again.
Optional Defensive Fire: You aren’t required to use your defensive fire if you don’t want to. For example, if your opponent moves a weak unit to provoke defensive fire from one of your units, you can choose to “hold your fire†and let the weak unit pass in order to be able to use defensive fire against a more dangerous enemy moving later in the phase.
Multiple Defensive Fire: If more than one unit has the option to take defensive fire, you can make one unit’s attack before deciding whether the other units also attack.