Early North Americans migrated across the land bridge from Asia during the Ice Age mostly likely due to a food/resource shortage
The Zhou Dynasty used the notion of the Mandate of Heaven to defend the overthrow of the Shang Dynasty. The "Mandate of Heaven" is an earliest Chinese philosophical concept, which was created during the Zhou Dynasty(1046-256 BCE). The Mandate of Heaven decides whether a ruler of China is satisfactorily righteous to lead; if he does not accomplish his responsibilities as an emperor, then he fails the Mandate and thus the right to be emperor. Zhou leaders appealed that the Shang emperors had become fraudulent and unfit, so Heaven commanded their removal.
Germanys resumption of submarine attacks and merchant ships in 1917
Keep soviet troops in eastern europe following wwII
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).
