They can't force checkmate
2 Knights checkmate is not possible: Proof
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The proof actually goes like this:
As shown in the diagram, checkmate is possible, but it cannot be forced.
1. White's last move was Nf7#
2. And where did this knight come from? -> Obviously from g5, e5, d6 or d8. These are black squares and from a black square the knight cannot control the f8 square. (This is important to remember).
3. What was Black's last move? -> Obviously Kg8-h8.
4. Since the black king was in check on g8, the penultimate white move must have been Nf6+.
But why did the black king move to h8? The square f8 was free as said above. If Black had played Kf8, the game would have been a draw.
Everything the other way around:
But KNN can force checkmate against KP, if the pawn is not too far.
https://www.chessgames.com/perl/chessgame?gid=1151993
First, take the following position
now, consider white forcing the king to the edge as follows:
Now a sequence of moves can be played to get a certain position as shows:
White would like to go to g6 with the knight to deliver checkmate, however knights cannot "waste" a move. Therefore, if you try to play a move to wait that controls the same squares, the best case scenario is a stalemate, or a draw. However, if you imagine white had a dark square bishop, it could move back along a diagonal to waste a move. Forcing the king into the corner whenever the white player needs it to be.
In summary, the reason 2 knights cannot checkmate is not that they are not "powerful" enough. It's the design and the nature of the piece that has to do with a very advanced chess concept that has to do with waiting moves. Because a knight cannot do waiting moves, they cannot checkmate a king.