Arimaa/Rabbit Advancement

An elephant, camel, horse, dog, and cat differ only in their power over and vulnerability to enemy pieces. Rabbits, however, are unique in two ways: a rabbit can score a goal, and it can't step backward. Since rabbits can't backtrack, rabbit advances may be risky, although the benefits often outweigh the risks. Except for a rabbit advance that will force a goal, rabbit advances are always a strategic issue, not merely a tactical one.

Exposed to capture
Unable to retreat homeward, advanced rabbits are often vulnerable to capture in away traps. Going fishing for rabbits would be slow, but when rabbits advance on their own, they have done part of the job for the opponent.

In, diagrammed at right, Silver's three advanced rabbits are doing nothing useful, and can't all be saved. Although material is even at the moment, at least one silver rabbit is about to be lost. As long as the silver elephant protects the other two, its mobility is severely limited. Silver is clearly losing.

Rabbit advances should be part of a larger plan. Rabbits might participate in a swarm, or might advance on the a-file to give friendly pieces greater mobility on the b-file. The problem here is that the silver elephant is only protecting rabbits, which is very wasteful.

Weaken goal defense
Rabbit advances also affect the area they leave behind. If one's home rank is left sparse, an enemy rabbit may have an easy path to goal. In, Silver had no way to block the western goal line, because there was nothing to unfreeze the c8 dog. No matter what Silver does from here, Gold can play Hc7wn ra7e Ra6n, forcing goal on the next turn.

By contrast, Gold has four rabbits still at home. The three advanced gold rabbits threaten goal right, left, and center, yet the four gold rabbits at home ensure that Silver would have to fight through more than one line of defense to score a goal.

Impede friendly pieces
An enemy rabbit can be pushed in any cardinal direction that there is an empty square, but there is no way to move one's own rabbit backward. Advanced rabbits can thus be used against their own friendly pieces, sometimes with devastating effect.

After 18s of, the gold elephant was cut off from the west, where the silver camel was thus a strong threat. If Silver keeps her own elephant and g6 horse in place, and also keeps the e5 gold rabbit blocked from the north and west, the gold elephant will have to maneuver through the southeast in order to go west. This will give Silver time to drag a gold piece to c6. Furthermore, the e5 rabbit will itself be at risk when the gold elephant leaves, although capturing it should not be a priority for Silver.

Multiple misplaced rabbits could prove even more disastrous, perhaps allowing for a complete blockade of the friendly elephant. Misplaced rabbits may also block a friendly attack or capture.

Advantages
Measured rabbit advances, however, can be quite effective even when there is no forced goal in sight. Since rabbits constitute half of one's army, various jobs could fall to them. Rabbits might advance to block enemy pieces, unfreeze friendly pieces, help control a trap, or threaten goal. A strong rabbit advance may end up doing all four.

Tactical goal threat
Once substantial material has been traded or become tied up in local fights, holes start to appear in each side's defenses. A formidable goal threat may occur at any stage of the game, but goal threats eventually become the central focus. In, Gold was way behind in material, but the a7 and h7 gold rabbits generated significant winning chances. Gold to move could threaten goal on either wing: Ed7e rf7n Ee7e would require Silver to use her entire turn on goal defense, leaving the silver camel or g7 rabbit to be captured. Alternatively, cc7s Ed7w hb7s Ec7w would require the silver cat or camel to occupy c8. The situation would remain precarious for Silver; with the latter move, Gold can force a goal in five turns.

If Gold presses the goal threats, the silver elephant won't have much time to move. Other silver pieces have also lost power; Gold's western goal threat effectively ties up both silver horses, and the silver camel could be captured or forced to the goal line if Gold pursues the eastern goal. Goal threats often outweigh a raw material lead, and may lead to a material gain even if the goal is stopped.

Goal threats also place great psychological pressure on a human opponent. Whether or not the goal could be forced with perfect play, a blunder by the defender could lose the game, whereas a blunder by the attacker would only give the defender a reprieve.

Strategic goal threat
An advanced rabbit can restrict the opponent's development, especially if it presents a strong potential goal threat. In, Silver had to carefully guard the western goal line against a gold rabbit on a7. Gold eventually lost shared control of the c6 trap, as the gold horse and dog were captured in a material exchange, but Silver never had time to capture the rabbit. With material depleted, blocking this western goal was costly for Silver, especially when Gold made another goal threat in the east.

Trap control
As forces thin, rabbits become more consequential. Despite Silver's material and alignment weaknesses, her dog-rabbit attack on c3 threatens a two-turn goal. Had a gold rabbit been on b3, any silver attack on c3 would have been slower and riskier.