Not really. Unless they decide to wipe out all life on the planet with massive nuclear strikes, nothing short of an armada is going to be able to take out *ALL* the military defenses of Earth. I think people underestimate the size of our planet.
All the military defenses which can inflict any harm on the aliens, I think should be the correct term.
Really it depends on how many, how capable and how advanced the aliens and their ships are
Yes, they can bombard us from orbit, but as we've seen from airstrikes, bombardments aren't always effective. Hardened underground bunkers can both withstand orbital bombardment and hide from the Alien sensors so that they won't know where to strike in the first place.
If they are prudent military strategists in the first place they will have scouted us out years before hand.
Eg; they send in satellites in orbit and small ships onto the planet to scout, barely detectable apart from occasional UFO radar contacts.
After studying our planets military powers, they send in two dozen large (star destroyer-like
) vessels to "warp" in around Earth, settling into orbit without any early warning by us. They take out high value military targets (ballistic missile bases, space and air bases, naval bases, carriers, chain of command) and infrastructure in the important countries first, who can field any credible resistance, and then move on to the lesser countries military bases and civilian infrastructure. That will be done with orbital bombardment/precision strikes and it doesn't even have to be DEW or even explosives. Rail guns firing cheap projectiles from orbit with resulting hypersonic+ speeds will be devastating.
There's nothing to protect population centers, so if the Aliens decide to strike there, the casualties will be horrendous. However, the Aliens will eventually run out of ammo before they can take out all of Earth's military, and they'll be forced to send in ground forces to secure resources to make more ammo.
Assuming they don't have power/nuclear generators to power their DEWs not to mention propulsion, warp drives etc
Even then they'd prolly have supply ships, not to mention reinforcements which can warp in?
I think what's important is that they can knock out our credible military defenses with a swipe. Sure there might be some assets left to us, but infantry, tanks, helicopters... even an aircraft carrier, won't be much use against orbiting space vessels.
Of course, it's also possible that those Aliens will precede their arrival by detonating massive EMPs that fry all computer chips in the world. In which case they may very well wipe out Earth's conventional military with ease.
Still, if they're here to exploit Earth's resources, they still need to send in ground forces, and that's when they become vulnerable. If the humans can sabotage the Aliens' supply lines, they can win a war of attrition since they're the ones with an entire planet at their disposal, while the Aliens have at most a dozen spaceships.
Their supply lines will be from the sky though, via ships. And humans armed with rifles and MANPADS, even tanks and mild air support will not be able to do much against the hypothetical defenses of a hypothetical alien harvesting vessel (re planet devastator from star wars
)
