# Checkmate all the kings #2

Following the rules created by Lord of dark in this puzzle, the idea is to find the minimum number of consecutive white moves to checkmate all the black kings (in this case just 1). You cannot make a move that would put white in check.

RULES

• You are playing as White and you can make as many moves as you want before Black's turn.
• During your moves you can take any black piece except kings.
• During your moves your king can not be in check position.
• At the end of your turn all the black kings must be check mate : if Black can make one move that ends with one king being safe, you don't win. Note that this one move can't be a king moving to a threatened position.
• One piece can be used in multiple checkmates (you don't have to take all the king, just to checkmate them)
• Edited my answer (and removed my comment as possibly too spoilery). Nice puzzle! Commented Mar 18, 2017 at 16:54

The minimum possible number of moves is 3:

xC6 (en passant), c7, c8=N

• OK, that's pretty damn neat. Commented Mar 18, 2017 at 20:36
• Wow, this reminds of classic puzzles by Sam Loyd. great answer :) Commented Mar 19, 2017 at 0:11
• Its worth noting that this first move is only legal if the black pawn got to where it was from a double move rather than two separate moves so I'm not sure if I would consider it valid without that context... Commented Nov 15, 2017 at 12:07
• The normal convention is en passant is illegal UNLESS one can prove the last move must have been c7-c5 (which is clearly wrong here) Commented Oct 2, 2020 at 4:17

Two possibilities for a four-move mate:

1. Kd7

2. Bb8 OR B x c5

3. Kc7

4. d6

OR, if the black pawn has just moved from c7 to c5, a three-move mate:

1. d x c6 (e.p.)

2. Bb8

3. c7

Proof that this is optimal:

• The only white piece which can guard the square a8 is the white-square bishop: obviously the black-square bishop can't, the king can't get close enough, and the pawn can't promote in time.

• So in order to get a checkmate, the pawn has to move out of the white-square bishop's path.

So the pawn must move.

• The only piece which can place the black king in check is the black-square bishop.

So the black-square bishop must move.

• Regardless of whether it checks from c5 or from b8, the square b8 must be guarded by a friendly piece, and there's nothing which can get there in a single move.

So at least three moves are required. This seems to be the intended solution; but if the black pawn has just moved from c6 instead of from c5, then four moves is optimal; see this version of my answer for a proof.

• Excellent answer. But not quite right! Commented Mar 18, 2017 at 16:42
• @DrXorile You mean it doesn't work or it isn't optimal? (I just edited in a second possibility, btw.) Commented Mar 18, 2017 at 16:43
• It's definitely checkmate! It's just not optimal... Commented Mar 18, 2017 at 16:44
• 3-move answer doesn't work for two reasons; one, it subjects White's king to check from Black's rook following 3rd move, and two, Black king can escape at Kb6.
– Rubio
Commented Mar 19, 2017 at 2:38