Arimaa/Rabbit Advancement

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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 can be risky. Even in the opening, however, advanced rabbits can aid stronger friendly pieces or impede attacking enemy pieces. The timing of rabbit advances is thus a difficult strategic issue.

Disadvantages[edit]

Exposed to capture[edit]

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Silver's advanced rabbits are no threat to goal, and the silver elephant can't defend them all.

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 this game, 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.

Advancing a rabbit often entails a willingness to let the opponent use up steps capturing it; if something else is going on, the opponent may not have time for the capture. Furthermore, no piece can be captured in a trap defended by its friendly elephant, so one attacking an away trap might safely advance rabbits on that flank. The problem here is that the silver elephant is only protecting rabbits, which is very wasteful.

Weaken goal defense[edit]

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With no rabbits on his back row, Silver can't block Gold's western goal.

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 this game, 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[edit]

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Unable to step south, the e5 gold rabbit blocks the gold elephant from moving west. (game)

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.

In the diagram, the gold elephant is cut off from the west, where the silver camel is 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.

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[edit]

Nevertheless, rabbit advances are sometimes necessary. 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.

Endgame goal threat[edit]

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Silver's superior army is limited by Gold's goal threats.

Once substantial material has been traded or become tied up in local fights, holes start to appear in each side's defenses. As forces thin, the game increasingly centers around goal threats. In this endgame, 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.

Gold's goal threats have weakened the silver elephant, which is far away and won't have much time to move. Other silver pieces have also lost power; the northwestern 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.

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[edit]

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The a7 gold rabbit will force Silver to keep the western goal line blocked.

If an advanced rabbit can't be captured quickly, it is a perpetual goal threat that must be blocked, perhaps restricting the opponent's development. In this game, a gold rabbit on a7 tied up several silver pieces in the northwest. 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[edit]

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The silver rabbit on b3 currently gives Silver control of c3. (Game)

When forces become thin in a particular area, trap control may increasingly depend on rabbits. Although Silver's alignment may look poor, Silver is the only one who currently owns an away trap. The b3 rabbit gives Silver full control of c3 and threatens a two-turn goal. Had a gold rabbit been on b3, Silver could not have created this threat.