I almost joined the Marines (but failed to make it for OCS due to health reasons). My dad's side of the family is a military family, with my grandfather being a career officer in the Army and two uncles also serving.

I think it's much deeper than "defend the country." For a lot of people, the military is the only ticket out of their bumfuck nowhere life. For some people, it's a chance to learn some self discipline and self confidence. For some people, it's a brotherhood of warriors they're part of. For some people, it's a way to get a college education. For some people, it's about defending their nation. Of course there are plenty of people who watch a Michael Bay movie and think they'll be some badass operator if they enlist.

I think someone like Smedley Butler might have viewed the military as a place that turns boys into confident men, teaches them valuable skills, instills discipline and self respect into them, and hooks them into a network of lifelong brothers.

War is not necessary for any of this, and he realized that when we go to war, he has to send his men to die so that people with contacts in Washington can get rich. He says in the linked paper that he suspected it when he was a Marine and finally realized it after he retired.