Complex sentence: On 8 September 1954, at the initiative of the United States, a military organization is created-SEATO-(equivalent of NATO in Southeast Asia) whose official goal is to form a "quarantine line" against Communist expansion.
Split into simpler sentences: On 8 September 1954, at the initiative of the United States, a military organization SEATO is created. Equivalent of NATO in Southeast Asia, its official goal is to form a "quarantine line" against Communist expansion.