http://en.wikipedia.org/wiki/AmbushI know wiki is usually bad but it gives a couple good examples. Ambush was used by both aggressors and defenders.
Whoever arrive in a region first should get an option to use this as they probably have the time to prepare for an ambush.
Also, it might be good to let ambushes set for sunset be more successful than ambushes set for sunrise meaning if you ambush people at night it is more likely to succeed than ambushing them during day time.
The one problem with setting an ambush in an enemy region is that they can easily check before you set up an ambush by using the region page.
Ambushes were used to take your enemies by surprise. By doing so you could lower your enemy's morale and make them panic.
