Man, there's a lot of pessimism in this thread. Mankind's not going anywhere just yet, folks. Those nukes are all just for show anyways. Besides, we likely won't ever experience interstellar war. Faster Than Light travel is impossible by our current understanding of physics. In fact, some would say that if an object that had mass managed to go faster than light, the entire universe, past, present, and future, would tear itself asunder. Then again, some theorists swear by intelligent design.
The point is, is that it takes time to get between stars. Light needs four-ish years to get from here to Alpha Centauri, the closest stable star system to Sol. We can't physicaly go that fast, so we have to use conventional means of getting around, like fusion reactors. Thus it would take us four and a half-ish years to get from here to there, AND THAT'S ASSUMING that we can get up 99.9% the speed of light. A more realistic estimate would be something close to 25-30%. So its going to take us tens, hundreds, maybe thousands of years to get from Sol to Alpha Centauri.
That means that your enemy (assuming Alpha Centaurians are hostile buggers) is going to have that much time to prepare for you and that much time to advance his technology. Your new, top-of-the-line battlefleet is going to find itself awfully outclassed by an order of generations once it gets to its destination. Additionally, the enemy doesn't even need a battlefleet of his own to kill you. Why, if he knew what vector you were coming in on, he could just toss twenty dollars worth of buckshot in your path and destroy your entire multi-gazillion dollar fleet. Why? Because if your ship runs into anything while its going 25-30% the speed of light, you're dead. EVEN THE SMALLEST PEBBLE could turn your big, burly flagship totally inside out. Just like a Triumph Spitfire doing 90 into a wall. You COULD hope that the debris from your ships (still going at relativistic speeds) would smash into the enemy planet, though.
So, basically, interstellar warfare is a nightmarish impossibility. Take comfort you fatalists! Mankind can't quite extinguish itself that easily. INTRAstellar warfare is still another thing entirely though.