When I've first found this thread I thought "nonsense...noninteresting" but some days passed and it still on the top! Now I've read it and actually seems interesting.
@first post: We also have R-7 missiles and other sub-orbital long range weaponry that could be easily modified to hit LEO or MEO objects. We also still have research on high intensity LASERs, Tesla-like weapons, my favourite EMP generators and others that could be handy in this situation.
@Artlav: it's true, if you want something why blow it up? In a realistic sense I also ask why would aliens want to extinguish us, but whatever. Also, strategically, ground operations are more probable if those aliens have a similar warefare technological level than that of us.
An important question they would ask themselves is were to strike, because there are not many resources for such large world surface we have (In this post I consider the invaders to not being much more advanced than us). Maybe the White House?
@TNeo: Splattering weapons are inefficient, but they cause a psychologic impact (you think that in the film is cool, but many people in real life would not like to see that).
About viruses and engineered diseases in general have interesting strategical applications in war and is true that if or victims are quarantined the aliens could very well attack again. These biological weapons, as chemical weapons (but no one ever think about this ones :dry

and radioactive (or "dirty") weapons, may also cause a big impact on people psyche because, in opposition to conventional weapons the kill slowly, in long term and may spread without needing to lose resources. Remember that many of the deaths caused by the Chernobyl Disaster were result of depression and stress of the "liquidators". The only problem with these weapons is that people may cover themselves on time if they are prepared (eg. hide in air-tight bunkers). Specifically the biological hazards are very versatil and be used in many situation: quick infection but very stealthy or of slow death but disabiliting and very prounonced
@Ghostrider: if anyone is thinking that alien forces will not be protected against or weapons with shields or CIWS then they think we are pathetic...
Maybe milk will burn them

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My plan in this case (expecting the aliens to have a technological level only slightly higher than us and that every country helps in the war) is to use the Moon as a control point because we will need to be able to launch spacecraft and on Earth that's too expansive. If it happened within the next 5 years or so, NASA could modify their Lunar Project easily (IMO). We could detect the alien forces before they reached Pluto's orbit so we'd have some time to prepare ourselves. There are a lot of abandoned as still functional Cold War Era Fallout Shelters and the like that could be restored and used if needed an evacuation. If aliens try to use asteroids against use then probably they would do it just to spare orbital bombardment systems what would indicate the limits in resources they would have. If they came here using just a Mother-Ship then the quickest we react the better. Large ships are problem so probes for reckoning are important to build a map of targets to aim (alien population clusters like resting rooms/dinner rooms or armament storages) to. Much probably there will be no force/electric field (too sci-fi) so it would be easy to hit the strategic points with experienced pilots or smart guidance systems in Nukes unless they use some kind of advanced interception rounds and that way we needed our LASER systems that could be deployed by satellites on high/geosynchronous orbits and overwhelming waves of attack and outnumbering. I believe that outnumbering of alien forces is feasible because for the long distances they would have travelled and the reason that would lead them to invade us (most probably planet resources depletion) would render them with lack of resources and units therefore they are limited unlike us in the present. It's inevitable that they will try to land on the surface most probably trying to divert in military bases while stockpiling our resources. For that we'll have to use conventional war (I don't see any way around). Finally wait, adapt to their technology and watch: "Optimism, Captain!".