Well, if an actual belligerent alien race decided to wipe us out, we'd be pretty helpless, yeah. They wouldn't invade, they'd bombard us with KEW's from orbit until we had no more governments or military, then swing on down and wipe up the rest of the resistance. But that's not the sort of invasion people like Phillip are talking about. They don't care about what's realistic, they care about what's cool to think about, and a movie-style invasion is so much cooler than what an actual invasion would look like.

However, if you're referring to such an invasion yourself, then I beg to differ.
We have good reason to believe that energy shielding would not operate like it does in the movies, but more like a material armor that doesn't wear down. But armor can be penetrated, and energy based armor can be penetrated a bit more easily (albeit with a bit more technology required) than physical armor.
Advanced forms of physical armor wouldn't be that much of a problem, as increased density means increased weight, so they couldn't have anything too dense. Anything that isn't incredibly dense is still susceptible to standard armor piercing munitions, so we're already prepared for advanced, alien, self healing armor.
Energy weapons are by their very nature inefficient, unless there are some super-advanced physics behind them, so we likely wouldn't see too many of those, and if we did, their efficiency would result in either low rates of fire or small ammo stores, both disadvantaged when compared to modern ballistic weaponry.
Biological weapons are relatively easily detected and defeated, as are chemical weapons.
Nuclear weapons would be counter-productive, as any such invasion suggests strongly that the invaders are trying to keep our infrastructure largely intact.
Advanced aircraft might pose a problem, but aircraft which travel too fast or bank too hard hurt the pilot, and so aren't really worth it. You can't dogfight when you're blacked out from pulling too many G's.
Electronic warfare would be the area in which they have the greatest advantage, but then, most militaries here are not overly reliant on advanced electronics, and those that are tend to understand the disadvantage that comes from over reliance upon that equipment, and train their troops to operate in the event of it's loss.

Another point is the size of such an invading force. The more soldiers you pack onto a ship, the more difficult that ship is to construct, maintain and fly. The larger a fleet is, the higher the odds of loosing a ship or two. I could see 2 or 4 million invaders at most being involved in this sort of endeavor.

Finally, a generation ship is not out of our current technological grasp, and is the most likely source of such an invasion. There is no real need that such an invading force be that much more advanced than us, assuming they're willing to spend a few generations getting here. Hell, they might even have 1980's level technology (that when I estimate that generational ships moved within our technological grasp, although that's just a rough estimate, I could be off by a decade or two, either way), in which case we'd still make mincemeat of them.

OK, Really finally now: Simply because a force has advanced technology doesn't mean they have extensive military experience and complex strategies and tactics. It's possible the aliens might rely on the shock & awe of their arrival, combined with WWII-era tactics to invade earth. Hell, it's possible they may not even bother doing any intel gathering beforehand, in which case they'd likely be relying on outdated information about our military capabilities.

Technological advances can be quite potent. A modern army of 100 could easily defeat an army of 1000 relying on 1000 or even 100 year old technology. But there's a point at which technological advances produce smaller and smaller returns. We've seen this ourselves in the past 30 years. Our greatest military advances in that time have not come from the R & D departments of big defense contractors, but from the experienced drill sergeants and officers managing TRADOC, devising and adopting new and unconventional warfare tactics to teach our troops.


This is very true. The only reasonable goal of such an invasion would be to subjugate and enslave the human population, and that's assuming quite a few things which aren't necessarily true (that the aliens practice slavery, that they don't have advanced robotics to do their menial work, etc, etc).

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It is also the reason that they would be unlikely to invade. What could they possibly want here... other than the sheer joy of killing us if you want to assume that they are like the aliens from "Mars Attacks".