[quote=mdk]Hypothetically, if you lined up enough high-energy plants and enough mirrors and built a big enough laser, you could use it as a defensive stand-off weapon. If it was up to me to attack such a planet, I'd approach from a different axis -- for instance if the laser is on Earth's equator, I'm coming in from the North Pole. Then again, if the planet is capable of building such a weapon, I don't think it'd be a likely target for an imperial occupation -- we'd either control it, and thus we'd be the ones building the laser, or someone else would already own it, and we'd be fighting them at a softer point (if at all).
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At the moment the theoretical "best" laser we can do with current physics and equipment is a bomb-pumped laser.

Nuclear bomb surrounded by mirrors designed to funnel as much power as possible into the laser emitter. The laser weapon ceases to exist moments after you set off the nuke, but the nuke powers a very powerful laser strike. But even with the best theoretical levels of that technology at this time, if you can make bomb pumped lasers you'd get better results just detonating the nuke inside the enemy ship.

(And contrary to popular belief, without a direct hit or VERY VERY close detonation, nukes as warheads in space is pretty close to useless, if the nuke detonates more than about a kilometre from the target vessel it's effectively harmless..)