Complex sentence: The damage to "Illustrious" was unable to be repaired at Malta so she was sent to the United States for repairs, 806 Naval Air Squadron remained in the Mediterranean joining another Illustrious class aircraft carrier.
Split into simpler sentences: The damage to "Illustrious" could not be repaired at Malta so she was sent to the United States for repairs. Meanwhile, 806 Naval Air Squadron remained in the Mediterranean, joining another Illustrious class aircraft carrier.