Atomic chess

In standard chess, the captured piece is removed from the board and the capturing piece takes its place. In atomic chess, a capture causes an "explosion" through which both pieces are removed from the board as well as all non-pawn pieces in the eight squares immediately surrounding the capture. Pawns are removed only when capturing, captured directly, or promoted. In case of en passant, the capture is considered to take place on the sixth rank square where the capturing pawn would have landed. Captures that result in the explosion of a player's own king are illegal, and therefore a king can never capture any other piece.[6][7]

Black to move: Win or stalemate?

	a	b	c	d	e	f	g	h
8									8
7									7
6									6
5									5
4									4
3									3
2									2
1									1
	a	b	c	d	e	f	g	h

With Black to move, this position would be a stalemate in variations of atomic chess that do not allow a king to move into check, but a win for White in variations that do not enforce the rules of check. In no version of the game would it be legal for the king to take the rook.

Threats to the king can be direct or indirect. Putting a king in check is a direct attack and is sometimes called "atomic check".[8] An indirect threat occurs when a player threatens to explode the king by capturing an adjacent piece. In most variations of atomic chess, players must respond to being checked in a similar way they would in traditional chess—by moving the king out of check, blocking the check, or removing the checking piece—but an exception exists when the checked player can win the game by exploding the checking player's king.

As the king cannot take another piece, it is possible to move the kings next to each other without a check occurring or to escape check. A losing player will often aim to position his king next to his opponent's in an endgame, as this is almost always a draw. If a piece or pawn can be blown up next to one king (without the other blowing up), or if one player can occupy the squares next to his king with pieces (or pawns) forcing the opposing king away, it is a win.

The variant of atomic chess played at the Internet Chess Club is one that does not enforce check at all, making legal any move, even one leaving the king to be captured directly in the next move.[7][9]

In all variations of atomic chess, a player wins by exploding the king of the opposite color without exploding their own king. This is usually done by capturing a piece on a square adjacent to the opponent's king or putting a queen next to the king which results in checkmate since a king can not take any piece.

In variations that require a response to being checked, it is possible to checkmate an opponent to win the game, creating a situation in which a king is directly threatened and cannot make a legal move. This is sometimes called "atomic checkmate".[8] It is likewise possible for the game to end in stalemate if a king is not directly threatened and no legal moves are possible.

When typical rules of check are not enforced, victory is only attained via the explosion of the enemy king. In these variations without check, the king may be forced into check and thus capture through zugzwang.

A common opening trap

	a	b	c	d	e	f	g	h
8									8
7									7
6									6
5									5
4									4
3									3
2									2
1									1
	a	b	c	d	e	f	g	h

A common response to 1.Nf3 in chess is 1...d5. In atomic chess this leads to a forced loss after 2.Ne5!!, threatening Nxf7. After either 2...f6 or 2...f5 White can play 3.Nd7!!. White's knight cannot be taken by any piece because of its proximity to Black's king, and Black cannot stop 4.Nxf8.

White has the initiative in atomic chess, and many openings begin with a quick attempt to explode Black's d-, e-, or f-pawns in front of the black king.[10] For this reason atomic openings are fraught with traps and Black may have to respond precisely to defend. Several common traps begin with 1.Nf3 followed by Ng5 or Ne5, forcing Black to start with 1..f6 to maintain rough equality.[2] Games between experienced players follow more traditional opening principles such as piece development, controlling space, and winning material.[10]

An endgame position

	a	b	c	d	e	f	g	h
8									8
7									7
6									6
5									5
4									4
3									3
2									2
1									1
	a	b	c	d	e	f	g	h

Black has connected kings and become immune to check. However, White can win by forcing the black king to be adjacent to the black pawn, then capturing the pawn.

1.Kc4 Kd4 2.Kb5+ Kc5 3.Ka6+ Kb5 4.Ka7+ Ka6 5.Qxa5

Black is forced to retain the connection, or the queen will chase the king and deliver mate.

As with atomic opening theory, a small amount of endgame theory and analyses have been published online by strong players.[11]

A common scenario in atomic endgames is to have two kings on adjacent squares. As a player cannot intentionally blow up his or her own king, a king can never capture another piece, including another king. Similarly, because a capture affects all adjacent squares, one king cannot be captured directly if it means destruction of the other. Moving one king towards the other is thus a strategy to achieve a draw. The player with the advantage must then maneuver into a position where a non-king piece can be captured that is adjacent to one king but not the other. If there is no piece that can be captured, the player with the advantage can promote two queens and force the opponent into zugzwang and therefore forcing the opponent's king away from his king. Creative use of zugzwang is a common tactic to force a win in these situations.[12]

The Internet Chess Club variation of atomic chess ignores check completely, and thus leads to different outcomes in certain situations. For example, a king and rook vs. king endgame that would be a draw under normal circumstances can become a forced win on ICC.