An army may march great distances without distress, if it marches
through country where the enemy is not.
We must beware of understanding 無人之地 as “uninhabited country.”
Sun Tzŭ habitually uses 人 in the sense of 敵, e.g. _supra_, § 2.
Ts‘ao Kung sums up very well: 出空擊虛避其所守擊其不意 “Emerge from
the void [_q.d._ like “a bolt from the blue”], strike at
vulnerable points, shun places that are defended, attack in
unexpected quarters.” The difference of meaning between 空 and 虛
is worth noting.