Why was the great wall of china considered a zone of transition

The Great Wall is a fortificated East-to-West line which historically has delimited the Chinese Northern border. Its primary purpouse was to defend the Chinese territory against the invasions and quick raids organized by the nomadic people from the Eurasian Steppe lands, located more to the North.

But rather than as a fixed border, it should be regarded as a transition zone which establishes a distinction between two different lifestyles, even nowadays: the farmer regions in the South with sedentary ways of life and the pasture lands in the North, where its inhabitants have nomadic lifestyles and work mainly in animal husbandry activities.

because the vietnam war was fought in an especially brutal and untraditional way