Writing political/military fiction?

I have no professional experience but my advice would be to have an idea of what you want to achieve, perhaps a message to your target audience.  Then just work towards that goal, it also helps to have a rough outline of your story.  Though not a necessity, Tolkien would start writing hit a dead end, and start from the beginning again.  Most important I would say is to remain positive and determined don't beat yourself up if you get stuck.  You should enjoy yourself you are putting a part of yourself on paper.  If it helps think of it as a spiritual experience.