Complex sentence: It was the headquarters for the 9th Infantry Regiment (Devine Elephant) of the 5th Division (ARVN), a US helicopter company and a US Special Forces/CIA base immediately adjacent to the air strip which had its own security perimeter within the main perimeter.
Split into simpler sentences: It was the headquarters for the 9th Infantry Regiment (Devine Elephant) of the 5th Division (ARVN), a US helicopter company and a US Special Forces/CIA unit. That unit was located immediately adjacent to the air strip which had its own security perimeter within the main perimeter.