When talking about joining the Military, I think, "Knowing what they are getting into" is a bit of an misnomer.
Personally, when I signed up at the age of 17 war was the last thing on my mind. They sell you you a lot of shit while you are being recruited some truths, some half lies and some gray area shit. My mind was on that signing bonus, free college, and that Va Loan (which I thought was a free house).
I think (know) the avg recruit fresh out of High School is thinking about getting some cash and getting some training (shooting, hand to hand combat, etc) and some other shit, not war.
Hell, sitting at the table w/ the recruiter and moms, my moms flat out asked him, what are the chances of my baby going to war? Dude was like man, hes gonna be brand new, so I doubt he goes. Not knowing how deployment groupings work, we accepted this and kept it moving.
The reality of going to war didn't set in until I was towards to end of AIT when everyone got their orders. The instructors were going around the room asking everyone where they were going to be stationed. They got to me and I said, "1st Cav". They were like oleeee shit, 1st Cav, thats rapid deployment!!! Will, yo ass going to war 1st! They thought that shit was funny as hell.
Cant lie, I was shook up for a lil bit when we got back to the Barracks. Crazy thing is, I got to my unit Jan 3rd, or 4th. Pretty much the whole unit was on leave due to the Holidays. A few days later they called everyone back because we got orders to deploy. A week later they were cancelled.