Right now I'm sitting in Portugal, Europe, typing this on a laptop which was designed in America and manufactured in China. And we're not at war with one another at the moment. I believe there may have been a few consequences to exploration that were far nicer than war, trade chief among them.

Not only that, if we've become advanced enough to finally decide we should genuinely spend our efforts on exploring space, by then perhaps a humanistic consciousness could have emerged rendering war irrelevant.

Sure, there are times of peace, and there are times of war.

But history tells us there likely will be wars.

Interplanetary warfare would be extremely resource intensive. You can't very easily occupy a planet.

Just getting an army from one planet to another would be ruinously expensive. Also, without some big leap in propulsion technology it's going to take something like a year for the invasion force to arrive. That gives the defender a whole lot of time to think about how he's going to respond and prepare accordingly.

Good point: The attacker loses the element of surprise and must maintain very long supply lines!