[quote=catchamber]
How would this interstellar invasion fare against lasers?
[/quote]

We don't really know, without knowing how that technology is going to develop. Kad's on the money with regards to our current understanding -- laser weapons are effective as countermeasures against certain sensor packages, because of how they gather and use data; but beyond that, as a medium for energy transfer, we just haven't figured out a way to make it worthwhile (not to say we never will). Famous examples are the Star Wars missile defense system and the ABL, both of which were rendered obsolete by ballistic defense systems (ie, iron dome). 

Based on our present tech, theoretically you could mount a mirror to the front of your ship and (assuming your reflectivity [albedo] is high enough), the laser is rendered harmless. Perfection isn't something we usually manage, so it's not like lasers are always going to be 100% useless -- but when you look at how stealth ships (air and sea) are designed to mitigate their radar signature, it's likely that 'lasers,' as we currently understand them, wouldn't be effective as space weapons. Not in this scenario anyway -- you could certainly conceive of many uses (melting comets, potentially pushing satellites, communications, etc), but yeh. 

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).