Can a single soldier ever be the piece that actually delivers checkmate, entirely on its own?
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
Yes, and more often than most players expect. With no other piece on the board, a soldier and its own general beat a bare general in 2,862 of 3,294 positions, in at most 10 moves. Every draw is a position where the soldier has already reached the last rank and can only shuffle sideways. The one partner it needs is its own general, which comes up and takes away the defender's file under the flying-general rule.
Why this is possible despite the soldier's limited reach
Checkmate never requires the mating piece to control every escape square itself, only that no escape square remains, by whatever combination of pieces achieves that. Against a bare general, your own general does much of that work: standing on an open file, it denies the enemy general that whole file, and the same flying-general rule often makes capturing the soldier illegal, because taking it would leave the two generals facing each other. The soldier only has to take away the last points and give the final check. Every case is counted on the proven endgame verdicts.
Where the draws come from, and the defended general
A soldier on the last rank has lost its forward move and can only step sideways along the back edge, which is not enough to build a net. So the rule of thumb is simple: a lone soldier wins unless it has already reached the last rank, and pushing it all the way down early is a real endgame mistake. Against a general that still has advisors the picture changes; there a soldier usually needs help, except in smothered positions where the defender's own advisors have already sealed off the escape points and the soldier only has to land the final check.
Related questions
Does the soldier need to have crossed the river for this?
Yes, it has to be close enough to the enemy palace to threaten a point inside it, which requires having crossed.
Is this the same idea as being smothered by your own advisors?
Only against a defended general, where a soldier mate is usually a smothered mate with the defender's own advisors blocking the escape points. Against a bare general no smothering is needed: the soldier and its own general do the work between them.
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