[The nine changes of warfare are as follows:] Do not encamp on entrapping terrain. Unite with your allies on focal terrain. Do not remain on isolated terrain. Make strategic plans for encircled terrain. On fatal terrain you must do battle. There are roads that are not followed. There are armies that are not attacked. There are fortified cities that are not assaulted. There is terrain for which one does not contend. There are commands from the ruler that are not accepted.
Sun Tzu, The Art of War, c. 450 BCE