How do cannons and soldiers combine for tactics near the river?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws for movement, checkmate and both repetition rulings: a side that forces a repetition by perpetual check or perpetual chase loses rather than drawing. The chase half has not been validated against the official casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it before publishing.
Direct answer
A soldier that has just crossed the river is often perfectly placed to serve as a cannon's screen, standing as the only piece between the cannon and a target it could not otherwise reach. Because the soldier is already advancing as part of the normal plan, the cannon threat it enables can arrive with little warning.
Why this combination appears so often near the river specifically
The river is where soldiers gain sideways movement and where the fighting typically first becomes concrete, so it is naturally the zone where an advancing soldier and a cannon aimed down the same file or rank first line up. A defender watching the soldier's advance in isolation can miss that it has just become a screen for a threat behind it.
What to watch for on both sides of the board
As the attacker, look for moments when advancing a soldier one more point would put it alone between your cannon and an enemy piece it is aimed at. As the defender, treat every advancing enemy soldier as a potential screen and check what is lined up behind it before assuming the advance is purely positional.
Related questions
Is this specific to soldiers, or does it work with any piece near the river?
Any piece can serve as a screen anywhere on the board; soldiers near the river are simply a common and easy-to-miss case because their advance looks routine.
Does the soldier need to have crossed the river for this to work?
No, but a soldier that has just crossed is the case that most often catches players off guard, since its new sideways options draw attention away from what it is screening.
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