Arimaa/Elephant Blockade

An elephant mobility advantage is most often procured by threats which keep the enemy elephant on defense. Sometimes, however, an elephant can be physically blocked. If an elephant is surrounded and can't simply push its way out, that elephant is blockaded.

The corners can be dangerous places for elephants. Gold might willingly expose a small piece to capture in c3 if such a capture would require the silver elephant to end the turn on c2. Early on, players discovered that some bots could be lured into a blockade this way. Once the elephant found itself surrounded, it would look for a quick escape, and thus could be tempted further by an empty c1; the blockade could then become stronger, as it is easier to smother a piece against the edge.

In, Silver took the bait and was strategically lost by the ninth turn. Due to the b1, d1, and c2 phalanxes, the silver elephant has no move at all. With the only functional elephant, Gold is in firm control. Silver could try to free her elephant, but Gold could ward off or capture any silver piece which approached the blockade. To keep it intact, Gold must continue to occupy c3, and thus should restrict most home captures to f3.

Rotation
Most elephant blockades are not as lopsided as the one above, which was only possible because the bot thought nothing of where it placed its elephant. An elephant even one square away from the edge will at least have a potentially quicker escape path.

Having his elephant stuck on g7 would be no problem for Gold if Silver kept this exact blockade in place. Until Silver moves a blockading piece, the strongest free piece is the gold camel.

Silver can, however, rotate pieces out of the blockade. If Silver now slides his camel and three rabbits with mh6s rh7s rh8s rg8e, those rabbits will block the gold elephant from h7, and the camel can move west. Although g8 is left empty, it is not an escape path; the gold elephant must stay on g7 to avoid being completely smothered against the edge.

Silver should try to form a phalanx to replace her elephant, which could then dominate the board. Gold must race to free his elephant; a gold piece, maybe the camel, must reach some place where it can pull away a blockading piece, thus allowing the gold elephant to push its way to freedom. Silver could make such a rescue harder by playing a move such as ce7s ce8s rd8s. If a path does open for the gold elephant to move west or south, it must do so immediately. Even if the gold elephant escapes, the rescuing piece may be lost, but that is better than having one's own elephant stuck and the enemy elephant free. Gold must throw caution in the wind to break this blockade.

For the blockade to remain intact without the silver elephant, a silver phalanx must extend to g5, so the blockade would use more pieces and be more breakable than one which stuck the elephant against the edge. Still, a free silver elephant could easily neutralize the gold camel, and the silver camel could stop any other gold piece. In the, the silver camel went to g5, rounding out this phalanx while continuing to protect the h-file.

Were the gold elephant on g6 and the silver elephant on g5, it would be a different story; a silver phalanx extending to g4 and f5 could be quite vulnerable, so elephant rotation might not be feasible for Silver. If rotation is not an option, the strength of a blockade depends on the free pieces. The opponent might attack on the other wing, or might try to break the blockade from the center; the blockader might then give up the blockade but capture a piece.

In any blockade position, a blunder by either side could be huge. Both sides must keep track of possible escape paths and even possible captures.

Cost
As with a frame, an elephant blockade may require too much material when one's forces are already depleted. The silver elephant appeared hopelessly stuck in, but Gold had to move pieces west to defend goal.

Elephants in middle
With ee4nn rc6e, Silver to move can block the gold elephant from the south and west; d7 would be blocked by a phalanx, and e6 would be blocked by the silver elephant, which would also defend f6 so that Gold couldn't capture anything there. Surrounded on three sides, the gold elephant couldn't flip any piece. The gold elephant might hope to escape via the east, but Silver could stop that with solid tactics, beginning with a fourth step of rh8s, which would allow Silver to block the gold elephant on both the g- and h-files.

With no clear escape path, a surrounded elephant should probably stay as close to the center as possible; this blockade would not be strong unless the silver elephant could rotate out, which would be much more likely if the gold elephant went east and thus marginalized itself further. Gold might instead create a western threat to force the silver elephant to abandon the blockade.

Friendly rabbit blockade
Unable to step backward, a rabbit blocked in the other three directions cannot be moved by its owner. When advanced rabbits of the same color are side-by-side, they might all get stuck in place, forming a wall which could severely limit the movement of other friendly pieces.

After move 48s of, well-placed silver pieces combined with five advanced gold rabbits to blockade the gold elephant, camel, and dog. In the east, four more gold pieces couldn't move at all. The gold horses were stuck in the south. For the next eight turns, Silver's elephant and eastern horse did all the work, pushing Gold toward complete immobilization. To avoid this, Gold sacrificed his elephant.

Such a blockade could be risky on a depleted board. In, the stuck silver rabbits threatened goal, and Gold walked a tight rope to keep them blocked while also defending in the east.

Blockading the gold elephant on g6 would often require the silver elephant to remain on g5, since a silver phalanx extending to g4 and f5 could be highly vulnerable. In, however, such a phalanx was formed mostly by stuck gold pieces. Gold might have hoped to get a piece onto e6 or f7 and then move his elephant through the trap, but Silver can now easily prevent that. Two advanced gold rabbits have allowed Silver to completely immobilize three strong gold pieces.

With a gold rabbit on a5 and the silver elephant on b5 in, the gold elephant was stuck in the northwest. If Silver can get an additional gold rabbit onto b5, the silver elephant could rotate out and dominate the board.

Partial blockades
Even if an elephant can't be fully smothered, it might be marginalized for several turns. With Silver's elephant boxed in, his horse-by-camel hostage is useless; the strongest free piece is the gold camel. The silver elephant could eventually escape, perhaps through the c6 trap, but the gold camel could make a capture in the meantime.

The Center
A semi-centralized elephant can sometimes be blockaded by a phalanx on a central square. Such a phalanx will occupy three of the four central squares; a strong piece on the fourth central square might protect and reinforce the phalanx. Silver's central blockade is strong, not only because of his camel but also because of his advanced elephant and horse. Even if Gold got his elephant onto d5 via Hc4w rd4w cd5s Ec5e, Silver could then capture the c3 rabbit by dislodging the c2 cat; Silver would then have a strong trap attack and goal threat.

This strategy requires great caution; if a central dividing wall does not hold, the pieces likely can't all retreat. This blockade worked because of the attack on c3, which the silver elephant soon rotated out of.

Swarm
Even in its own home territory, an elephant is not always safe from a blockade; a swarm can sometimes blockade an elephant next to its home trap. In, Silver exploited the bot's susceptibility to elephant blockades.

The frozen gold cat has become part of the blockade holding in the gold elephant. Gold might try to change that with ha3s Eb3w rb4s Ca4e, but an elephant usually can't escape a blockade via the edge of the board; in this case, the c4 horse could then push the gold cat back to a4, immobilizing both the cat and elephant, as b3 would now be blocked by a silver phalanx.

As things stand, the gold elephant might eventually escape to the center via the second rank, but then Silver could make captures in c3, and the advanced silver rabbits would become strong goal threats. Silver's swarm has put the gold elephant in an impossible situation; a home hostage-holder must be wary of such a swarm.