text: There are bits and pieces throughout the responses that are correct.  \n\nThe Marines are within the Department of Defense.  However, within the DoD they are administratively attached to the Department of the Navy which is led by the Secretary of the Navy.  A Marine 4-star is on the Joint Chiefs of Staff (one is currently the Chairman) as is a Navy 4-star.  So, for all intents and purposes they are their own branch of the military but due to traditions and history they have not been given their own department within the DoD.  Although, I believe there is  probably a large majority of senior Marines that would like their own Dept even though falling within the Dept of the Navy is really only administrative.\n\nIt's just the history that at one time they were the Naval Infantry.  There is no reason to not think of them as their own entity.