An old Marine told me that Marines guard Marines from the other side. And when one of their brothers is being threatened, the Devil Dogs (a.k.a. Marines) will “go wild on them” for eternity.

Yes, but what about the sisters? Do the Devil Dogs protect them, too? What about the female Marines whose nude photos were posted to a Facebook group where comments ranged from raunchy to suggestions of violence?

Do female Marines count in the Devil Dogs lore?

The questions arise as the Defense Department begins an investigation into recent revelations about the Facebook group Marines United, which the Associated Press reports was composed of active-duty and retired male Marines along with some Navy Corpsmen and Royal British Marines.

Some of the nude shots were grabbed from Instagram, which mostly prohibits nudity. Others were shot surreptitiously. Most were passed along a testosterone-rich grapevine. More than two dozen active-duty women in the photographs were identified by their rank, full name and location. Needless to say, the women were horrified to learn that they had been sexually objectified by their peers.

One said the scandal had ruined her Marine experience and that she wouldn’t reenlist. One active corpsman said he’d seen the photos on the Facebook page, which also provides news and support, but wasn’t interested and skipped over them. He didn’t find the collection surprising, however, likely given his generation’s comfort with nudity in all its forms.

The young women who knowingly had their photos taken apparently thought that viewers would be of the women’s own choosing. One can imagine, however, that a libidinous corpsman (pardon the redundancy) who discovers a picture of a semi-nude or nude female Marine might be inspired to share it. Isn’t “sharing” the operative term in today’s narcissistic, show-and-tell-all culture?

The difference and the distinction, however, is that the Marines United boys club basically stole the images and used them without the subjects’ consent. Marines being Marines? Or are they guilty of something more sinister, potentially deserving court-martial?

To the civilian mind, the answer is rather simple: The Pentagon, now fully infiltrated and indoctrinated by modern feminists, has decided to put women in combat (thank you, President Barack Obama). Therefore, women must be treated as men.

But what about the vice versa? Must men be treated as women? That is, should they be trained to be more “sensitive”? If so, can you simultaneously create sensitivity in the desensitizing, killing culture that breaks down an 18-year-old’s humanity and instills in him an instinct for extreme brutality?

Put another way, how stupid are we?

There’s a reason we say in times of great peril, “Send in the Marines,” and it’s not because of the few brave, committed women among them. But try to find someone in today’s military willing to say so.

Older vets with nothing to lose will sometimes open up. Two of my regular Marine correspondents, “Jack” and “Russ,” both of them Vietnam vets, explained the culture that creates killers and how this environment isn’t conducive to civilian norms.

Jack, who told me the afterlife story, is my brother. Russ is a retired Methodist minister who counsels veterans navigating post-traumatic stress disorder. Neither they nor I intend to justify the Facebook group but rather aim to illuminate the mind-set that might have led to it and the misunderstandings that create havoc.

“Hollywood makes this s--- up,” says Jack in his best “French.” “I never saw a Marine shed a tear for lost buddies. Now in the Middle East, these guys have f------ breakdowns and unit ceremonies and all that s---. In Vietnam, your buddies put you in a body bag, a chopper flew your dead a-- to Da Nang, and off you went on a jet back home.

“It ain’t Hollywood. It is stinky, bloody, sweat-soaked, soil-your-britches killing and being killed. You push that cr-- down so far in your guts that it comes out 50 years after the job is done. That’s PTSD.”

Russ explains the culture in somewhat more polished terms.

“Marines embrace the warrior archetype more than other branches. The shadow of this is patriarchy, misogyny and brutality. We are trained to be killing machines, deadening all emotion except anger. We’re told we don’t have the luxury of sensitivity, so we objectify everything, including women.”

Still, he’s optimistic, saying that we need to return to “the embodiment of the hero archetype in the medieval knight. Aggressiveness can be coupled with honor, nobility and compassion.”

Maybe so. But knights typically didn’t joust with women, which may be the most salient inference. That said, chivalry has a place here. An apology to the women who exposed themselves to the few, not the proud, would be appropriate — both as gesture and punishment.

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