[PvK]

The main problems I see are if this was a game where victory was based on score (which is a problem in itself); then the issue that the receiving empire does not have to agree; and that it can cause a period of at least one turn of unexpected peace for the enemy, who if he had a state of peace with the recipient, will suddenly not be able to intercept or follow through on attacks, at least for a turn (after which he can choose to declare war on the recipient or not).

AFAIK however, none of these were the case in the example this thread started out about. In which case, as I've explained twice, I don't see this as much of an advantage in general, and I do see it as disadvantageous in several ways.

The other cases can cause some problems, like in the "surrendering to the AI" part. Really the attacker should be allowed to keep attacking the target worlds, at least until given a chance to decide whether to declare war on the "new enemy" or not.

Having said all that though, there are a couple of related tactics that players should decide whether they are clever and legal or inappropriate (I could go either way, myself):

1) When another empire is mopping up a defeated empire, getting it to surrender to you, even though you weren't really attacking it.

2) When some of your ships or colonies are in danger, gifting them to other empires, to protect them from your enemies. (At least the recipient gets to choose whether to accept or not.)

Of course, surrender can simply be disabled during scenario setup.

PvK