客主之勢
The Strategic Positions of Attacker and Defender
兵有客之分,有主人之分。客之分眾,主人之分少。客倍主人半,然可敵也。負..定者也。客者,後定者也。主人安地撫勢以胥。夫客犯隘逾險而至,夫犯隘..退則刎頸,進不敢拒敵,其故何也?勢不便,地不利也。勢便地利則民自..自退。所謂善者戰,便勢利地者也。
In warfare there is the role of the guest (attacker) and the role of the host (defender). The guest requires a larger share of troops; the host requires a smaller share. When the guest has double and the host has half, then they can match each other. [The host is] the one who is established first. The guest is the one who is established later. The host secures his ground and controls the situation while waiting. The guest must force through narrows and cross over dangerous terrain to arrive. Having forced through narrows [...] retreat means death, yet advancing they dare not confront the enemy. Why is this? Because the strategic situation is unfavorable and the terrain is disadvantageous. When the situation is favorable and the terrain advantageous, then the people will naturally [...] and naturally withdraw. What is called skillful warfare is making the situation favorable and the terrain advantageous.
Notes
The guest-host (客主) distinction is a fundamental concept in Chinese military theory. The 'guest' (客) is the invading force that must operate on unfamiliar ground; the 'host' (主人) is the defender with knowledge of local terrain and established supply lines. Sun Bin establishes that the attacker needs at least a 2:1 numerical advantage to compensate for the defender's inherent advantages.
