Before I start, a little bit of background: I am a practicing student of karate and I train in an Okinawan style of karate called Shuri-Ryu.
Like Redleg said, a lot of martial arts schools in the US have lost the martial, or "military" aspect of it, and teach mainly flashy techniques like high, jumping, spinning, something, kick.
If you're looking for a martial art to train in as a prelude to going into the service, look for a school that is straightforward, little to no flashy techniques, is disciplined, and works both standing and ground-fighting.
So, as a general tip in that area, try to stay away from karate, taekwondo, kung-fu, etc. I'm not saying that they're not good, I love shuri-ryu, I'm saying that it's quite hard and few people are fortunate to come across a teacher that understands actual combat and prepares students for it.
A few months ago, I was able to work with a man named Avi Nardia, a reserve Major in the Israeli IDF who served in a counter-terror unit before becoming one of Israel's top hand-to-hand instructors. One of the things that he worked with in our school was called Kapap, something that I had never heard of before that night. Kapap is a system of combat principles that adds on to existing martial knowledge to improve upon the combative efficiency of the martial artist. These were developed for military use. If it's something that you're interested in, be sure to check it out. Avi sells DVDs that detail these principles.
Krav Maga is a great place to start, just make sure that you find a good teacher that knows what he's doing.
Also, look into jiujitsu. Not any jiujitsu school though. Make sure that they teach combat jiujitsu, not sport jiujitsu. The sport variety will limit what you are taught, but the combat jiujitsu is taught without rules and will include many techniques that are used standing up as well.
Hope at least some of this was helpful.