Conversation so far:
who is currently the highest ranking military officer
General Mark A. Milley is currently the highest ranking military officer
what is general mark a. milley's title
General Mark A. Milley is the current Chairman of the Joint Chiefs of Staff
when did general mark a. miley become chairman of the joint chiefs of staff
General Mark A. Milley has been Chairman of the Joint Chiefs of Staff since October 1, 2019

Question: who does the chairman advise
Self-contained rewrite: who does the chairman of the joint chiefs of staff advise