Freelance tends to be a problem because, in order to be a correspondent, you need to get access, which includes access to the country in which there happens to be a war. You also need to be credentialed, which means you need to work for a “legitimate” news organization. The Pentagon decides what is a “legitimate” news organization. P.J. O’Rourke wrote about all this in Give War a Chance.
For this angle, I’d talk to a couple people. A military recruiter would be a start, but they tend to lie. Another good person would be Gorgon Heap. Keep in mind that, as a military journalist, you’re military. You do not get to choose what stories you get to cover; the military tells you what information you can release and what you can’t; you can’t even be guaranteed access into a war zone. You also have to maintain proper military standards. That means you must meet physical standards, whatever training standards your service requires, and unit readiness.
Something else I’d like to say here is that if you’re in a war zone, you’re subject to the same rigors that a soldier is. You must obey the laws of the country where you happen to be assigned; you might be out in the middle of nowhere for God knows how long; you still have to deal with censorship of your stories; and you may very well be in physical danger. Correspondents can and are killed and kidnapped.
Before you do anything, I suggest you read everything you can get your hands on by and about war correspondents. You might also talk to some journalism professors (Columbia’s journalism school is very well-regarded, so you can start there) and get some good perspective from them.