From a warfare standpoint it is a joke. I was in the Navy and was stationed with the 2'nd Marine Battalion on the USS Saipan and have also worked with many marines in the civilian world as well. I have many friends that are ex-marines. One of my best friends is an ex-marine. At my place of employment I work with 6 ex-marines.
There's two phases of training. In the first phase they break you down and make you see yourself no longer as an individual first but as a member of a team with a specific purpose in mind first, an instrument. You are an individual only second to that, the job you have to do - your mission - takes foremost precedence over everything else. In basic training you refer to yourself not as by name but by "this recruit", you do -everything- in unison with others, march to every area on base as a unit, never going by yourself unless it is to medical, etc.
In phase two, you learn that you can overcome any task thrown at you. You learn that your mission is to kill. You are made to think you can overcome any obstacle and believe nothing can defeat you. You are a warrior with pure, yet focused aggression, a killing machine.
Mindfulness has nothing to do with being the most aggressive, dangerous instrument of death-dealing on earth.
Whatever an enemy might do to an enemy, or a foe to a foe, the ill-directed mind can do to you even worse.