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, 28 October 2007
relevant text:
Reinforce the gates
Battle: Even if you control no units at the current battlefield: Target an unopposed unit there. Move the unit to a target battlefield where it would be opposed.
the question:
My opponent attacks province A with unit X. I defend province B with unit Y. My opponent, the attacker, chooses battle at province B to be fought at first. I pass, he passes, the battle resolves. Next he picks battle at province A. I play Reinforce the gates on unit X, moving it province B's battlefield where it gets opposed by unit Y. What happens after battle A resolves?
The rulebook states, that defending units stay at the battlefield until the attack phase ends, and that exactly one battle happens at each battlefied.
Thank you for your time.
[Mod edit: Thread title. Please make them descriptive. Thanks! -ZF]
Martin Lund (rules official), 2 November 2007
If no battlefields that haven’t resolved a battle remain, then all units still at battlefields return home, are bowed by resolution if they were attacking or allied, and all battlefields cease to exist.
If one or more battlefields that have not resolved or ended a battle remain, repeat the battle procedure starting with Choose Battlefield.
As noted in the Comprehensive Rules - Battles: After Resolution entry.
Battle A ending is no great shakes. Once all the Battles are done, Defenders and left-over Allies and Attackers all go home, as noted above.