One thing a lot of folks outside the United States often don't understand is the way the military system works. Since the draft was abolished in 1973 we have not had a conscripted army; it's been only volunteers. (Note: When men turn 18, they must register for the draft. You can do that at the Post Office. If they should need to reintroduce conscription in case of war, they've got the system ready to do so.)
What this means is that the army (let's just refer to the whole armed forces as the army to simplify things) needs to attract people to join or there won't be an army. So army service in the US is not particularly onerous and it provides a good number of benefits. (Of course, you have to risk getting shot, but probably 90% of soldiers are not in combat units, and even combat soldiers' risks are not all that high, when you figure that there are like one and a half million people in the army and about 500 have been killed so far in Iraq.)
First, it pays better than minimum wage and provides room and board and health care for you and your family. Second, they pay for your university education, I believe at the rate of one year of college for one year of service. Third, if you have any sort of aptitude, you'll get training in something sort of useful, driving a truck or communications or electronics or some other skill. Fourth, "1996-2000, Corporal, US Army" looks a hell of a lot better on your CV than "1996-2000, Burger-flipper, McDonalds".
One other thing the army does for some people is it gives them a sense of achievement. Hey, if you make it through basic training, you've proved something, because not everyone is tough enough or smart enough to do that. If you get promoted, hey, great, you know you deserved it. They don't pass out free promotions. And you get to go back to the old neighborhood wearing the cool-looking uniform. You also get a sense of camaraderie and fellowship, and since many Americans are patriotic, you get to feel (quite justifiably) that you're somebody important and virtuous because you're helping to defend your country and your fellow citizens.
I noticed today that the army is running ads encouraging people to join using the slogan "Earn dignity, honor, respect." That's what they're appealing to, and interestingly, that's exactly what street gangs promise their members. I'll bet that slogan works on a lot of people, since an army is really just a sophisticated street gang. It's got its soldiers, its turf, its hierarchy, its camaraderie, its violence, its symbols and uniforms, its discipline, its feeling of belonging, its probationary period that must be survived, its emphasis on honor and respect and duty and responsibility, the common feeling that you mustn't let your comrades down.
One thing the army is generally known as is the least racist major institution in America. Hell, a black guy made it all the way to the top, and success in the army depends exclusively upon your merit and competence, for obvious reasons. Who cares if your buddies are black or white? Your life depends up on them and you'd better get used to that. Most people do so remarkably fast, I've read.
There's a European stereotype, though, which says that American soldiers are all poor blacks and Hispanics. Now, it's true that army service tends to appeal most to the lower-middle classes, but you see all ranks of society (except the California and East Coast wannabe non-Americans) in the army. I'm not going to look the stats up; if you guys want to check me on this you can, but I have read that blacks are overrepresented in the army as a whole (the US is like 11% black, while the army is more like 18% black) but underrepresented in combat units (which are like 9% black). This, by the way, was also true in Vietnam. The legend that blacks were killed there in disproportionate numbers is simply not true.
Don't ask me why this should be. My guess is it's got something to do with relative educational levels of average blacks and average whites. The white person quite likely went to a better school than the black person and so is more likely to qualify for combat duty. Of course this isn't always true and there are thousands of exceptions.