“You can be sure of succeeding in your attacks if you only attack places which are undefended.”
Certainty comes from hitting the unguarded; do not gamble against prepared defense.
Chapter 6 converts its routing doctrine into a guarantee: you can be sure of succeeding in your attacks if you only attack places which are undefended. Sure — the strongest word in the book so far. Sun Tzu offers certainty nowhere else, and offers it here on the strictest condition: certainty is a property of target selection, not of effort, courage, or execution quality.
A sales team is judged on logos — so it attacks the most defended companies in the market, where certainty goes to die against incumbency and procurement moats. A rival redefines the territory: underserved segments, neglected geographies, the product gaps nobody defends because nobody considers them markets. Same quarter, same commission plan: one team wins heroically sometimes; the other wins boringly always, and its "boring" revenue buys the later war.
Only attacking the undefended does not mean only attacking the easy — undefended points can be hard work that nobody bothered to contest. Nor does it forbid eventual contests for defended ground; it sequences them. You earn the right to besiege the fortress by owning everything around it first — at which point, Sun Tzu notes elsewhere, fortresses tend to open on their own.