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.
if I have a personality with Iron Tetsubo *negate all force penalties* and no followers would they still be affected by Three stone river and be dropped to 1 force?
Also would a force bonus from Shout of Defiance ad to their force it is was played after Three stone rivers is played?
Terrain Battle: Before this battle’s resolution, reduce the Force of each Personality at this battlefield to the Force of his highest Force Follower, or to 1 if he has no Followers.
So, it says "Before this battle's resolution." That means the effect happens after the Combat Segment. You aren't going to be playing any copies of Shout of Defiance (a Battle Action) after the Force reduction effect is applied.
As to the Iron Tetsubo, it says, "Negate all Force penalties on this Personality," and the Rules say,
Bonuses and penalties are commonly referred to by a mathematical expression such as +1F or -1 Personal Honor. The terms “gain,” “increase” and “give” for bonuses, and “lose,” “reduce” or “suffer” for penalties, are also used.
...
Any change in a stat’s value is considered a bonus (if increased) or penalty (if decreased), with the following exceptions:
Ref: Comprehensive Rules - Stats
So reducing your Personality from say, 6F to 1F involves a -5F Penalty, which would be Negated by the Iron Tetsubo.