I am so confused on this question…
They countered it because they ruined the land in every way possible which made it hard even for their allies from South Vietnam to support them. Napalm just burnt down forests and habituated areas and harmed a lot of civilians directly, while things such as agent orange destroyed the environment and poisoned the ground, the waters, the food, and basically anything else. It caused much more harm to the people than it did to help them.
Simply, it was an old region + part of the 16 Mahajanapadas, in modern day it is known as south Bihar
