埶 here is said to be an older form of 勢; Sun Tzŭ, however, would
seem to have used the former in the sense of “power,” and the latter
only in the sense of “circumstances.” The fuller title 兵勢 is found
in the _T‘u Shu_ and the modern text. Wang Hsi expands it into 積勢之變
“the application, in various ways, of accumulated power;” and Chang Yü
says: 兵勢以成然後任勢以取勝 “When the soldiers’ energy has reached its
height, it may be used to secure victory.”