You can trigger this quest by walking south, past the location where the Qunari patrol is attacked by abominations, along the path that leads to the southernmost section of The Wounded Coast area in Act 2.
Approach Lieutenant Harley to hear an appraisal of the situation. Both Aveline and Isabela have interesting reactions to this quest, so you may wish to bring them.
You have three choices:
Fight alongside the guardsmen. Players who favor diplomatic or aggressive speech responses can inspire these allies before the fight begins. This is definitely the easiest option.
Walk away. You can return to complete the quest later, but Aveline, if present, will express mild disapproval.
Attack the raiders without support from the guardsmen. This is difficult, but not intractably so. Aveline will deprecate this choice: why can't her guards contribute to the fight?
Note : There are seven(7) traps which are standard (20 cunning to detect and disarm - 100 XP/trap = 700 XP total) and two(2) Complex (30 cunning to detect and disarm - 150 XP/trap = 300 XP total). If fighting with the guards you have no control over them, they will set off the majority of traps, costing the player the experience from disarming them (1000 XP). There is a single Complex (30 cunning to open - 150 XP) chest behind the raiders.
Hawke has cleared The Wounded Coast of a dangerous group of raiders and assisted the guard. Evet of Evet's Marauders will attempt to exact revenge during a future visit (Act 3).
ps3xbox360 In unpatched versions of the game the player may repeatedly talk to Lieutenant Harley, with each end of the conversation netting friendship/rivalry points with Aveline.
pc As of 1.03, if the party leave the Wounded Coast in act 2 before completing this quest, it will glitch and won't complete. This happens even if the quest has not yet been received.(confirmed in 1.04)
xbox360 In some cases, if you fail to finish this quest during your first run through the Wounded Coast in Act 2, one of the raiders will stare at the battlefield, non-moving, non-targetable and the quest will go unfinished.
pc 1.03, Disarming the first group of traps to the west sometimes leaves one or more traps "flashing" as if you haven't disarmed them, and you will be unable to click them again. However, the traps are inactive as they should be, and XP is earned normally.