A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 30 October 2007
When EXACTLY are units opposed? Does it happen at assignment? Here's the real question. If I target Bayushi Hisako with bayushi Saya then assign Hisako to defend at a province with an attacking army, triggering Saya, then seppuking, does this trigger Hisako's reaction to dishonor one of the attackers?
Reaction: After Hisako is destroyed: Dishonor another target Personality who opposed her when she was destroyed, or who performed the action that destroyed her. The target cannot be rehonored this turn. Bow him if he is Crab Clan.
Thanks for your time.
Chris Fuchs (rules official), 30 October 2007
Opposed
Refers to a card, side or army at a battlefield that has one or more units in the side opposing it.
You are opposed when you assign to a defending army if there is already an attacking army present.