John Adams wrote that a patriot must be a "religious man." The Christian's faith calls him to civic duty - knowing the needs of neighbors and meeting them, demonstrating faith with works of charity. The Christian patriot, after all, spends more time washing feet than waving flags. And in flags he sees symbols, not of military or economic might, but of the common good of the specific people a sovereign God has given him to serve. That's patriotism in its place, a patriotism worth singing about - and even dying for. (Charles Colson, The God of Stones and Spiders, Ch. 39: On Waving Flags and Washing Feet)
I have had a very difficult time answering people why I joined the Marine Corps. Not because there is nothing to say, but because there is so much to say. There are many factors that go into that decision. And in a world of attention spans that last as long as the average marriage, I've had to answer as concisely as possible. The shortest answer is love. A slightly more elaborate answer is that I am a follower of Christ. As such, I have become a servant to all. In the Marine Corps, I can serve the entire country, and as an officer I can serve Marines, too.
I've also been asked, "why fight and possibly die for a corrupt and sinful country?" To which I reply, "Christ died for the ungodly." (Romans 5:6)
God demonstrates his own love for us in this: While we were still sinners, Christ died for us. (Romans 5:8)