Rulebook states that no ship movement is necessary for PDS to fire when that PDS owner activates system containing enemy ships (in PDS range).
Where does it say so ?
Yes, you can fire on your opponent's ships in that case. If the activated system is within PDS range and contains ships that are not yours, you can choose to fire on that system.
For the rules lawyers at your table, movement is a mandatory step in the tactical action sequence. You can choose to move zero ships, but you still need to execute the Movement step. When your opponent announces that he is not moving any ships, he has "finished moving his ships."
The purpose of that line in the rulebook is just to emphasize that PDS Fire (step 3) comes after Movement (step 2.)
Actually, no you can't.
Please allow me to quote a few excerpts from the core game rule book to clarify:
Page 10: "Except for the first step (the activation itself), each individual step of the Activation Sequence is only resolved if the condition for its resolution applies or is initiated by the active player. A player, for example, may activate a system to produce new units there during step 7 but does not necessarily have to move any ships into the system during step 2. Or, a player may activate a system and move ships into the system, but if the system contains no enemy ships, there is no Space Battle during step 4, etc."
Page 11: "3) PDS Fire - After the active player has finished moving his ships into the activated system, enemy PDS in range may fire at the active player's fleet."
Page 29: "When a player activates a system in range of an enemy PDS unit, the owners of any enemy PDS in range may, after the movement step of the activation sequence, fire once per PDS at any units in the system owned by the activating player"
And finally, from page 11 again: "2) Move ships into system - After you have activated a system, you may move friendly ships (within movement range) into the activated system. Only movement into the activated system is allowed."
Ergo, IMO one is not allowed to fire PDS on an enemy system that has been activated by the activating player if they have not actually moved any ships into the system during that same activation. Ships that were already in that system, have not moved at all, because they were already in that system, therefore the condition for firing enemy PDS units does not apply. Same goes for ships which are being built in a space dock during the same activation. They are immune from enemy PDS fire, even if in firing range, because the production step comes after PDS fire step. Therefore those ships may safely enter the system, or - by logic - they do not enter the system, but are rather built in the system.
To sum up, if a player activates one of their systems that contains ships which do not move into that system, but have already been in the system, then the PDS fire step is not triggered as there was no actual movement in step 2 of the activation sequence. The condition for PDS to fire simply does not apply in that instance, as it would only be resolved if there was actual movement into the system during that same activation.
Edited by Dwntn, 26 July 2014 - 02:53 PM.