Rulebook page 12:

"The active player declares which of his units in his Battlefield (if any) are going to participate in the attack.

Question:
Can defenders be assigned if there are no attackers? (for example, to take advantage of Scout keyword)

I believe the answer is no. Maybe if your opponent declares a zone to attack and then doesn't declare attackers, which I can't think of a reason for doing. But otherwise, if no one declares a zone, there is no battle, and therefore no chance to declare defenders.

There must be a zone attacked for combat to start, and I believe there must be declared attackers in order for it to continue to the assign defenders, but I can see a case where attackers were removed (one way or another) prior to declare defenders step and we have no specific ruling to guide us on whether Combat continues or ends immediately. Send it in to James.

It is simple. Defenders only damage attacking units, so, if there is no attackers, you don't do any damage, and scout keyword only works if damage is applyed and that unit is alive, so no discard.

Nevertheless, dont know if its possible to asign defenders if there is no attackers, but with the rulebook in one hand, you can asign defenders even if there is no attackers. Maybe something allows the attacker to move units in a future.

No, they don't have to deliver damage to have their keyword initiate. They simply need to survive combat which is to say make it through the end of the apply damage step.

I can see the intention, but if an attack is not declared against their zone, then no.

SiliconSicilian said:

It is important to not apply too much pre-judgement about how card games should work, especially since W:I is unusual in that "Declare Target of Attack" and "Declare Attackers" are so explicitly separated.

Dr.Cornelius said:

Based on the rules combat only hppens when attackers are declared... so you could use Scout only if that happened.

If you get rid of all attackers between "declare attackers" and "declare defenders" (with tactics for example) then you will be able to use Scout.

Maybe a suggestion for the multi-player rules. But you can't use "scout" if there was nothing to survive.

If you mean there must be combat initiated and they must have participated and remained in play, then you are correct. If you mean that there must have been attackers and damage done, you are incorrect. Combat has nothing to do with damage assigned or applied in this game, it is specifically the 5 steps the game works through. If I play Demoralize and have toughness then all damage was reduced or canceled, but my Scout unit survived through the damage application step so is considered to have survived combat and its keyword initiates. This is not any conceptually different than my playing Pilgrimage before the damage assignment step removing your lone attacker to your hand. My Scout survives through the stages and so his keyword resolves.

The important bit here is he must participate, IOW he must have been declared during one of the attacker/defender declaration steps, and when Step 5 resolves, his keyword then initiates (this would be before any player actions could be taken since a keyword is a constant effect). The original question seemed to ask if you could declare defenders if there were no attackers and I think the general consensus has been no, if there are no attackers there is no one to defend against. So any scout using defensive players are going to want to wait till the player action window after declaration of defenders, before they start removing all attackers from play.

Defenders cannot be declared if their zone was not declared as the target of an attack. There may be times when you would want to declare an attack on a zone without any attackers but I am betting those situations are VERY rare. I will admit that I have not read every card and there could be times when it is beneficial to attack a zone with no attackers, I just can't think of any.

I am NOT saying that I assume how this game is supposed to play. You will note that I said, "can see the intention," as in what you are trying to do with your mini-combo. However, I do know how the rules for attacking and defending are clearly stated...and your combo just doesn't get the chance to work. Others have added further intelligent points to support this.