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, 12 August 2009
From here
quote="Remi" Can I play Crushing Attack during a battle action segment if my army has 1 or more strength and without any opposing unit?
quote="Zen Faulkes" No. There must be an army.
Does the above mean that Broad Front suffers the same restriction? (ie must have an defending army at the second province?)
Broad Front
Battle: Target two battlefields that have a unit you control. Move all units at the first to the second. At the resolution of the second one's battle, if the attacking army exceeds the defending army's Force by more than the combined Strength of both Provinces, the battle destroys both Provinces.
Crushing Attack
Battle: You may only play Crushing Attack if your army currently has more than twice as much total Force as the opposing army. The battle action phase ends (resolve any terrain, if applicable), and this battle is now resolved.
Alex Jones (rules official), 13 August 2009
From here
quote="Remi" Can I play Crushing Attack during a battle action segment if my army has 1 or more strength and without any opposing unit?
quote="Zen Faulkes" No. There must be an army.
Does the above mean that Broad Front suffers the same restriction? (ie must have an defending army at the second province?)
Broad Front
Battle: Target two battlefields that have a unit you control. Move all units at the first to the second. At the resolution of the second one's battle, if the attacking army exceeds the defending army's Force by more than the combined Strength of both Provinces, the battle destroys both Provinces.
Crushing Attack
Battle: You may only play Crushing Attack if your army currently has more than twice as much total Force as the opposing army. The battle action phase ends (resolve any terrain, if applicable), and this battle is now resolved.
Absent stats: If a card’s type does not have that stat, the card does not have that stat either – the stat is absent from the card. If something in the game needs to know the value of an absent stat, the value is zero. If a side at a battlefield has no units, its total army Force is treated as an absent stat. Absent stats do not receive bonuses or penalties.
Broad Front does not care whether there are units in the opposing army; it can still destroy the two provinces if your army has sufficient force.
However, note that Crushing Attack was printed when the rules were slightly different. The ruling for Crushing Attack is specific to it, and has never applied to Broad Front. It is not precedential.