Copyright (c) 1963 Sid Sackson

This game is played on the following board:

STACK - A set of one to five stones of either color. Ownership is decided by the color of the top stone.
The stack size is equal to the number of stones it has.
FIRST TURN - Before the first turn, both players switch two or more stones to achieve an asymmetric board position (symmetric tactics can achieve an easy draw for the second player).
TURN - On each turn, each player must do one of the following actions:
Moves one of his stacks on any orthogonal direction, exactly the same of cells as its size, possibly landing over another stack.
Stacks, while moving, can jump over other stacks without affecting them.
A move can be made with only an upper part of the stack (that sub-stack moves the number of cells equal to its own size).
CAPTURE - If, after landing, the final stack size is greater than 5, all stones in excess of 5 are removed from the bottom of the stack. Enemy stones are removed, while friendly stones go to the player's reserve.
Use one of his reserve stones and place it on any cell (occupied or not).
If the cell is occupied, the stone is placed on the top (any excess over 5 stones is also removed from the board).
GOAL - The player that is unable to make a valid move, loses.
An example

Black's turn. Black has 4 reserve stones, White has 2. Black position is better, he has more reserve stones than white.

He can play a reserve stone at b6 taking that stack. White can attack f2 by moving g3-g2 (f2 cannot move to g2). But black can reply then with b6-f6 protecting his f2 stack.

An hexagonal variant of Focus is Crossfire with the following extra rule: [author words] "the capacity of a cell equals the number of adjacent cells!" Corners will hold at most a stack of two, sides at most three or five, and cells of the inner area at most six. This gives rise to a strategy not possible in Focus: moving big stacks onto 'low-capacity' cells, corners in particular. Thus considerable numbers of prisoners can be made and reserves created on fixed target cells. In Focus for instance one can never make prisoners, nor create reserves, by moving a stack onto a vacant square. In Crossfire, moving a stack of five onto a vacant corner renders three men.