Through the magic of coincidence, some of Loghain's troop movements have been made public. Using this information, go "investigate" the status of Loghain's troops.
After the quest has been accepted, a new location named Civil War will be added to the World Map. Upon arriving, you will discover that a group of Loghain's troops are attempting to seize Bann Telmen's land. With the help of the Bann's men, you can show them who's boss fairly easily. Focus fire on one enemy at a time so your NPC allies are freed up to do the same.
Return to any Chantry representative for a reward.