Divide and rule

separate in order to control, strategy of keeping a people or enemy separated into small groups in order to dominate them and prevent effective resistance

In politics and sociology, divide and rule (derived from Latin divide et impera) (also known as divide and conquer) is a combination of political, military and economic strategy of gaining and maintaining power by breaking up larger concentrations of power into chunks that individually have less power than the one implementing the strategy. In reality, it often refers to a strategy where small power groups are prevented from linking up and becoming more powerful, since it is difficult to break up existing power structures.
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