There was a good summation about this a few weeks ago which I learned from also.

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There's no centralized gov/legislative power over the armed forces.

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Troops and equipment are organized four or five co-equal commands. If it were a family tree the President and Commander in Chief is like the elderly grandpa and the four or five commands mentioned earlier are his independent adult children; the troops and equipment are the grandchildren/cousins.

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The adult children are off doing their own things with their families in different countries. The adult children may or may not go with whatever Grandpa says.

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The military is mighty. The military is separate and decentralized away from the three branches of government.