The Physics of Space Battles

Hmm...
I thought about this a while back, and I figured that a space conflict would be something like this:
1: Build orbital platforms that can accelerate a 10 ton mass to 1/10 the speed of light (in both directions).
2: set them in orbit around the moon, so they are always on the opposite side from your target planet (Orbiting once every 28 days.)
2.5: Give them a nudge so that they orbit a quarter of the distance around the moon, and are pointing at your target planet.
3: Fire off a dozen rounds.
4: Contact survivors and demand unconditional surrender.

Now, with intersystem war, things get dicier, but I imagine that planets would still be impossible to defend. If, for example, you can create a sort of missile that accelerates to 1/10 the speed of light in the four years it spends moving from Alpha Centari to Sol, and then fires off thousands of one-ton stealthed iron masses, you're doomed.

Against a planet with an established ecosystem, yes.
Against a planet best stripped of it's resources and constructed upon, no. Essentially, the planet will be the same, minus any major landmarks and/or stable climates.

Missiles would likely travel much slower than kinetic rounds, and ocne they run out of fuel, they could no longer change course, so they could be using simple kinetic ponit defense systems and possibly just cause a missile to run out of said fuel o destroy.

A Heavy kinetic slug on the other hand would be VERY difficult to stop once its in motion, not to mention hit when its moving within even something like a tenth the speed of light(it has to be detected, tracked and point defense needs to be fired off... if these are ships that are relatively close to each other, then it would be near impossible to stop