I am the commander of an invasion force. We have travelled across the stars to parasitise a small, blue planet somewhere on the Western Spiral Rim of the Milky Way. Specifically, my comrades and I have come for its water. There’s a lot of it. This “Earth” (such a foolish name for a world whose surface area is 70% water) is a rich prize.
Because I am not stupid, and because I understand that a) tactically, high ground gives you a major advantage and b) there is no higher ground than low orbit, I begin my assault by making a note of all the major gathering points of the indigenous population. Usefully, these are lit up at night.
Then I start throwing rocks at those population centres. They don’t need to be massive. Just big enough not to burn up on their way through the atmosphere. Something the size of a skyscraper, tiny in terms of the masses of rock and metal that swing around the sun in tandem with the blue world, would have a pretty appreciable effect on a major urban conurbation when it’s travelling at several miles per second. I don’t even need to be that accurate. Shockwaves and airborne debris would do most of the work. Then it’s a simple mopping-up operation.
Or, if I’d rather start a harvest of the water without causing genocide, I could land my forces in the middle of the Atlantic and Pacific, and set up seaborne facilities that would be difficult to detect until it was too late. I could have a defence grid in place before the aboriginals have a chance to organise a meaningful response. Maybe I’m already at it. You’ve heard of the Bermuda Triangle, right?
The one thing I would unquestioningly not do is land a ground force on coastal zones of high population without immediate air support, and then force them to fight inland in order to start a refuelling and harvesting operation from the sewers. The sewers, for fate’s sake. This would afford me the barest trickle of the resources that are clearly available. You know, all the blue stuff I can see from orbit? The stuff I came hundreds of light years to get at? Why would I fight an expensive and potentially ruinous land war against natives who, quite rightly, are going to be pretty honked off at what I’m up to? Like I said, I’m not stupid.
I am the commander of an invasion force, and I laugh at your Aaron Eckhart, just before I throw an asteroid at him.
EXCUSES AND HALF TRUTHS 
Good post. Real disappointment, that film. The first hour was great – scary, gripping, and living up to its publicity as being about the band of brothers (and one sister, natch), but then someone grafted on a Hollywood second half, including soppy familial relationships, impossible weapons tech and a mad race to destroy the Big Thing. Real shame.
If you’re advanced alien species, you’d think the artificial synthesis of water, which you require for- ya know!- you’re own life would be higher priority than slightly more advanced robotics and guns.