Qin, Qi, and Yan, although nestled in the southern border, surrounded by tall mountains and dense forests, laying on complex terrain.
However, at the boundaries dividing these countries, it's flat as a pancake. Even if there were mountains and forests, they have long been trampled flat.
One step forward, you will enter enemy territory.
Both sides of the border have their respective buffer zones. If I reserve five hundred feet, then you must also spare five hundred feet. If you only leave four hundred and ninety-five feet, I will advance five feet.
This is what is meant by the phrase "fight for every inch."
The so-called border, it's only about an inch.
Intrude my inch, and we are in life-or-death combat.