The cornerstone of the special forces group's capability is the a highly trained team of 12 special forces green berets.

Operational Detachment-Alpha (the A-Team)

Explanation:

According to my research on military operations teams, I can say that based on the information provided within the question the cornerstone of their capability is the Operational Detachment-Alpha (the A-Team). They are the main fighting force of the Green Berets on execute nine doctrinal missions such as  unconventional warfare, counter-insurgency, counter-terrorism, special reconnaissance, etc.

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