This question originally appeared on Quora. 
By Jon Davis, Veteran of Operation Iraqi Freedom, Sergeant in the United States Marine Corps
There are periods of intense pride mixed with a sense of entitled arrogance, followed by periods of massive disillusionment, followed yet again with a calming pride in your achievements and the meaning that they had. At times, we feel like the saviors of all humanity, at others we question if anything we have done or are doing is right, and then there are periods of intense resentment toward the civilian population for not taking part in what we are experiencing.
Boot camp is among one the most important periods in a Marine's development. It places within him a sense that he is expected to do important things, far more important things than could be expected from other eighteen year olds. This is all happening during one of the most intensely stressful periods of your life, where you are kept isolated from contact with your family and friends and taught that everything you were before entering the Marines was weak and lacking any real value until you too are a Marine. Cults are made this way too. I'm just saying. But in all seriousness, the psychological transformation of boot camp is a very intense and intentional effort by the Marine Corps to make warriors who will be able to fight and kill out of kids who have just barely left high school. From the point that you graduate boot camp, you will be different and have parts of the Marine Corps culture as part of your psyche.
Besides this is the prolonged exposure to the Marine Corps once you're in. Everyday you can't be at war, yet the Marines seem to be. Most of our time is spent either on deployments or in training, when we aren't in one of these two situations, we are constantly being reminded that we will be soon. There is the constant reminder that there are forces outside the States that are working very hard to kill us. There is a saying "Complacency Kills," and this, over the course of years, has another effect comparable to constant sense of paranoia and mistrust. Imagine a mild form of paranoid schizophrenia.
Along with these periods of "downtime" is the constant barrage of nitpicking at everything from the way you walk, the way you wear your uniform, even the distance of the little rank insignia from the edge the collar. A good Sergeant can spot that a rank insignia is not centered or aligned at thirty feet. This is part of the grind of Marine Corps life that over months whittles away at you and almost makes you want to go back to Iraq. You also deal with regular inspections to make sure that every piece of uniform item you own is perfect. Your medals hang at exactly the right place, your rank is centered in exactly the right spot, your shoes and boots are completely clean and shiny. This and hundreds of other things, all measured to within an eighth of an inch. This is part of a Marine Corps mentality to always look your best, perhaps to project that air of nobility and respectability from the general population, but this crap costs money! You're given about $250 a year to replace uniform items, but easily spend $1000 during post deployment inspection season and around the time of the Marine Corps Ball just getting to the point that you look presentable. So the next time you see a Marine in that nice coat and blue pants with the pretty white hat, you make sure to know that he paid for that in more ways than one.
After this is the constant training. From time to time, you will go out to the field and have some interesting lessons on camping trips. It feels very boy scouty with guns. Kind of fun actually, but then there are the trips to Yuma where you sit for months in the worst place imaginable in America. Seriously, I hate Yuma. It was much worse than Iraq. However this training makes the real thing much easier to deal with. Still though, you are in Yuma.
And finally the actual deployments. You spend months in a war zone where most of the time, things are quiet, mixed with short burst of chaos where you fall back on training and hope that your leadership is in control. It is the long term of it that gets at you. You spend seven months actively considering that you might be attacked at any moment. You miss your families and think about not seeing them again as a very real possibility. This happens in two ways, you think about yourself being next, but you also think about them too. I don't know if this is common, but I was also always concerned that something had happened to them, car accidents or something. There are no greater odds of this happening when I am deployed as otherwise, but that paranoia did give me some additional stress. Can't really explain that. As I said before, you are always thinking about death, particularly your own. I know that college hipsters write poems about death and others may fantasize about its nature, but they aren't in any real danger. Some of us really like living. Those cliches about "Embrace your death" or "accept that you are already dead" are silly lines put in movies. The fact is that you have to try and bury that emotion down as often as you can and only years later confess that you had them. Some nights you go to sleep in a cold sweat considering this stuff. It isn't really that you are in that much danger at any given time, it is that you are there in a dangerous place for months on end, with tons of time on your hands to think thoughts you probably shouldn't. In all honesty, statistics showed that it has always been more dangerous to be a police officer in New York City than to be a Marine in Iraq during the worst part of the war, but they get to cool off at night and see their families. I think that it is the long term effects of these deployments that cause so many of us to go off the deep end. Now you are surrounded by people and things that you can't stand after a few months, and you are stuck there for more. Returning home isn't easy either. There is a damaging mental effect that takes quite some time after a deployment to overcome that comes with long term deployments. I went into detail on this in How do Military Veterans feel when they return home from combat? It might be worth your time.
The last major thing that you go through is getting out. Now you are a regular civilian again, but not really. Remember that I described the Marine Corps as a place that builds a cult-like sense of schizophrenia and devotion to purpose, while still instilling a sense of pride that can't be recreated. Sometimes you look back and think about all the things you endured, from the minor ones like inspections and getting constantly yelled at to losing friends and never seeing your friends and family and that you are just so happy to get out. Other times, you are constantly annoyed at how much your new civilian counter-parts whine about their problems and can't ever be relied upon to get the job done with any sense of rationality or as a team. The self-centered mentality of individuals really gets under your skin after this. Of course everyone isn't like this, but the bad ones stand out greatly. You also get seriously tired of hearing the anti-war protests and people advocate all sorts of cuts for defense from people who have never been in. You get very angry at those who speak out against the military or who have never been a part of it. I know that it is very complicated in that the military serves to protect the rights of the common American to say and do those things which give value to life and drive our nations democratic process, and these are things that have to be said. But coming from some people it feels ungrateful, and as if coming from the mouths of the ignorant. The military is placed on a pedestal of gallant and noble defenders of American freedom and way of life, yet when the pressure gets to be to much for a few of them, then the military is little more barbarians. You spend a long enough time in the civilian world and you begin to forget the things about the Marines you didn't like and start to gain back that pride in a subtle way. It returns to you little by little when you tell people that you were in the Marines and without telling anything else they say "Oh" and though they don't really know what that means, they know you have done something pretty special.
Basically, as my fellow Marines pointed out there are highs and lows. There are peaks and valleys which most people are unaware of. The image that most people have of Marines are as they are depicted in the movie "A Few Good Men."
"The Marines... are fanatical."
"About what?"
"Being Marines."
Some people suppose that we are simply killing machines or simple men who have no greater thoughts other than the destruction of lesser men. We aren't. We are aware of our situations and have nothing more than time to reflect on it. The experiences we go through greatly shape our entire lives. The other Marines we share these experiences with form a culture of Marine, and together we produce a very old and eccentric fraternity of combat and honor.
When I was asked to answer this question I wanted to make it meaningful, but even in my very long answer I found myself unable to put into words everything that it meant to me. It was likely the most important and transformative event I have experienced in my life, and I wanted to do it justice.
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