The Pentagon is where military preparedness is fostered. In our system of government, the military is subordinated to civilian control, which is as it should be. They are not, as in so many countries, our bosses who maintain that position through fear. However, there are factions in this country that would like to see the military defanged, and, if possible, disbanded.
How convenient it would be for Anarchists, Islamists, and any nation states that would like to see us rendered defenseless. It would also please those pacifists who believe that if we are just nice, the rest of the world will be nice back. But most of us know better.
One function of the military establishment is training: training of young future officers who will have command duties down the line. In this training, the military provides another function that is in short supply in the rest of America's governing institutions: long-range planning.