take advantage of the enemy's unreadiness, make your way by unexpected routes, and attack unguarded spots.
20. The following are the principles to be observed by an invading force: The further you penetrate into a country, the greater will be the solidarity of your troops, and thus the defenders will not prevail against you.
21. Make forays in fertile country in order to supply your army with food.
22. Carefully study the well-being of your men,