A member of a small defensive force of soldiers

Guerrillas.

Explanation:

After WWII Argentina's leadership alternates between periods of military and civilian governments.

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The president of the United States cannot be a naturalized citizen - the actual phrase is  that the president of the United States must be a "natural born citizen" - meaning that they must be born as citizens and not naturalized later (being born a citizen and naturalization are two ways of becoming a citizen)