In military matters, this stratagem means: When a strong country wants to attack a more distant, weaker country, they must pass through a neighboring country. Therefore, before advancing to the attack, they must first pacify the neighboring country to secure its permission to borrow its roads for the long march.
In Go we only have two sides, the opponent and ourselves. There isn't any “friendly neighboring ally”. Applying this stratagem to Go, we can only say that you must first find a way to stabilize one battlefield before opening another one, if you want to gain profit in both places.
Basic Figure: Taken from the 1982 First “Summer Holiday Mountain Villa Cup” Tournament. The game has just about entered the yose stage (Black to play). The position just slightly favors Black, but there are a lot of points still to be had in yose, and it is difficult to quickly and simply settle the position. But there are a couple of things of which we can be certain: that the only direction in which we can play is a in the lower left corner, and that the
yose in the vicinity of thelower right has not yet been fixed. How we settle these two places will directly affect the final result of the game.
Being of such great significance, they are worthy of serious consideration.
Diagram 1: Black 1 is a big yose play, and at the same time it eyes an attack on the White group. We can say that it is very much an ideal move in its own right. But in the final analysis this move is not very bold. White's atari at 2, cap-turing the stone on the second line, is also extremely large. When Black attacks with the keima at 3, White plays keima at 4, pushes at 6, and then keimas at 8, a very good move! Here he has basically for-med a living shape (because Black cannot peep at a - if White plays de
at b then the Black stone at a would not be able to escape). The osae of Black 9 was largest. Osae at White 10 on the upper side was also worth more than ten points yose. Up to here, Black has about 80 points, and White has at least 75 points, with Black to play. The game has become so close that it is difficult to predict who will win. Black fails.
What would be the result if Black changed to play the yose on the right side? Please take a look at Diagram 2.
Diagram 2: A Black hiki at 1 here would also lack boldness. White 2, taking the left side, is also very big.
It is hard to say what the gains and losses are for these two moves.
When Black plays hasami at 3, White plays osae at 4, and there is no big problem here. Black's kosu-mi at 7 is not very useful. About all he can get out of it is the chance to play tsuke-hiki with 9 and 11 in sente. Tsuke at 13 to watari is very valuable, but White gains sente to nobi on the left side at 16. Sagari at 18 is tesuji. Black can only play
wari-komi at 19 to capture, while White wataris with 20 and 22, making considerable profit in yose. Black has clearly lost the battle.
Figure 1 - Actual Game Continuation: Black was certainly not in the lead, so he couldn't just play a sim-ple endgame; but, if he could just make a little profit at White's expense, he would be all right. So Black turned his attention to whether or not he could reduce White's corner a little and put a little pres-sure on him, or settle the position in this vicinity in sente so he could take the yose play on the left side.
This way, Black could be satisfied.
Black's hasami at 1, probing for White's response, was the important play at this point in the game! If White passively played 2 as osae at 7, then Black would atari at 6, then take sente to play at 25. This was Black's predetermined plan. Not to be outdone, White connected at 2. De at Black 3 was one of the keys to getting this game on the road to victory. White could not osae at 5, or Black would play at 4 and White would have to connect at 7, and the stone above 4 would be cut off. So White blocked from above at 4, abandoning four stones and looking for furi-kawari. The de at 7 was the second key for this game. (For the variation with the sim-ple play at 10, see Diagram 3.) White had to block at 8. Black cut at 9.
White's nobi at 10 was strongest. Black cut again at 11, and White had no choice but 12 (for the counterattack variation, see Diagram 4). Now Black could not be greedy or he would wind up losing. Warikomi at 13 was a calm move! Unconditionally capturing four White stones, and making not a little profit. Then hasami at White 18 capturing two stones was a big play, but out of this Black gained sente to play the big yose at 25. In this campaign, the exchange of Black capturing four White stones and White
capturing two Black stones was about even. The key was that while settling this part of the board, Black gained precious sente to make the big yose play at 25. Black was able to gain profit on both fronts, firmly establishing the lead.
Diagram 3: If Black just turns with 7 as here to capture four White stones, it would be a little slow. After White played sagari at 8, he could take sente to make the yose play on the left. Now rescuing one stone with 11 would be dangerous. White could cut at 12 and extend at 14 in sente, then
start a semeai with the hasami of 16. This result is disadvantageous for Black.
Diagram 4: If White does not play sagari with 12 as in Figure 1, and furthermore if he tries a semeai, the result is not good for White. Black would use the special property of the corner in the variation to 27, when White would not be able to play into the corner without connecting at 28 first. Then the hane at 29 turns it into seki. A seki result is obviously not good for White.
Diagram 5: When White played sagari [12 in Figure 1], if Black were too greedy, thinking of killing White, then things would be different. White can use guru-guru-mawashi tactics. Black sinks into a difficult situation and is captured.
Diagram 6: The good point about Black's cut with atari [11 in Figure 1] is that unlike attacking directly [shown here], there is the exchange of a and b in this diagram. With this two-move exchange, Black's clear profit is that later in yose, he only needs to play sagari at 1, White 2, and when Black ataris at 3, White's three stones would be caught in damezumari. This shows just how ingenious Black's play was in this corner.
Conclusion: First Black cleverly settled the position on the right side, gaining sente to reach his goal of playing the big yose on both sides of the board. The play throughout the campaign was tight, especially the ingenious sequence after the de at 7 in Figure 1. Not only did he gain profit in yose, but he also prevented White from coming away from the fight on the right side with sente. This gave Black the success arising from the stratagem “borrow a road to send an expedition against Guo”.