“If we wish to fight, the enemy can be forced to an engagement even though he be sheltered behind a high rampart and a deep ditch.”
You can compel engagement by attacking something he must relieve, regardless of his fortification.
Chapter 6 reverses the fortification problem: though the enemy shelter behind high rampart and deep ditch, he can be forced to an engagement — by attacking something he must relieve. The verse is the ancestor of every modern "asymmetric" move: you never break walls, you make the walls irrelevant by threatening what the walls were protecting.
A national retailer cannot be beaten on stores — the ramparts are sunk costs, prime locations, decades of leases. A challenger attacks cash flow instead: same-day delivery on the categories people buy weekly, the ones whose margin funds the real estate. The retailer must respond outside its walls, on ground it never chose, with an infrastructure it cannot build quickly.
Forcing engagement is not baiting into traps for sport — the compulsion targets something genuinely vital, and your own exposure is real while you threaten it. The verse also cuts against the impatient: the forced engagement usually arrives after the threat has been established, not in the same move. Point at the thing he must save, and let his necessity do your maneuvering.