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, 22 February 2010
I was told that if I play ambush, then sneak attack (or have a naval personality), then a legal action, and my opponent plays reinforce the gates, the resolution of this sequence not only stops the action taken by that personality but also negates the ambush. Is this the current ruling? If so, why?
Ambush: Limited: Target your unbowed Personality: Create a battlefield (not at any province). Assign your Personality's unit to attack there. Assign another player’s target unit to defend there, even if its Personality is bowed. Other units will not move there. Fight a battle there (after this action's resolution). After the battle ends, lose 5 Honor.
Reinforce the Gates: Reaction: After a player announces a Battle action before the Defender’s first normal opportunity to act or pass: Its effects do not happen.
Sneak Attack: Reaction: After engaging: The Attacker has the first opportunity to take a Battle action or pass during this battle. Play then proceeds in turn order from him.
Thank you for your help!
Zen Faulkes (rules official), 22 February 2010
I was told that if I play ambush, then sneak attack (or have a naval personality), then a legal action, and my opponent plays reinforce the gates, the resolution of this sequence not only stops the action taken by that personality but also negates the ambush.
Often, "I was told that there's this ruling" often means that there is no such ruling. This may not be malicious, but just someone misremembering or mistaken.
That seems to be the case here.
Advanced search reveals this to be the only thread in this forum that has both "Ambush" and "Reinforce" in it. So as far as I can tell, there is no such ruling and you were misinformed.
Reinforce the Gates does not negate Ambush.