Complex societies came first as during the human evolution, the early developed primates started as hunters and gatherers. Later when they began to settle in places for a long period, they started discovering new vegetation and various things related to it such as, how they grow, what is required for the growth, etc.
All this later turned into farmingpractices as settling at a place permanently required a fixed source of nutrition and farming was one of the solutions to these problems.
The answer is the North Vietnamese took over the South Vietnam spreading communism Southward and this proved the domino theory (if one country come under the influence of communism, the surrounding countries would follow).
