Do not attack cities that are members of alliances we are not at war with unless they have attacked you first. Only attack NPC cities, non-allianced cities, or those from alliances we are at war with.
The swap over to the new alliance "Plague" has begun. When you log in to Evony, click the Alliance button and you'll be told you are not a member of an alliance. But the invite for Plague should be there for you to accept.