What main idea drove nations to form organizations such as nato, seato, and the oas? \?

 The correct answer is : The main idea that led several nations to form this type of organization was to prevent the invasion.

Explanation:  These organizations are collective defense, where members are committed to protect any other country that suffers an attack. After the Second World War the countries of the North Atlantic region created NATO in order to protect themselves from the aggression of the USSR. In Southeast Asia SEATO was created with the same NATO guidelines in Europe but now for Asia, these nations united to face the communist expansion, but they were not as successful as NATO.

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