Alien invaders has dominated Sci-fi since the ages of War of the Worlds. There are many variants of this classic theme from post-invasion theme featured in Falling skies, to humanity desperate fight againts invaders in first days of they arrival featured in movies ID4 or Battle:Los Angeles. You could also played prolonged specops campaing based on UFO:Enemy Unknown or fan-novel X-Com:Unknown Menace. lets look how to game into this Theme.
This could mean only one thing. Invasion. -Star wars Episode 1
First important thing is say when your invasion will happen - you can easily play original War of the Worlds in late 19 century or even between or after World Wars, but i think most typical setting will be modern day invasion.the option is even viable in near or far future where first contact can get bloody.
Human forces will be always easy, no scifi gear (Unless your are playing scifi background say 2020+)
classic Orbats and vehicles to support your army should be again modern machines. So basicly if you want play Battle: LA scenario you will propably field modern day United States Marines which could be backed up by modern vehicles like M1A2 Abrams, HMMWVs, Cobra or other tech that US uses today.
As always designing Aliens can be tricky, because when you design them you must first ask few question about who they are. first thing you need to figure out is how they armies are organised they can be Coalition of many races (or perhaps they have robotic slaves) or Single powerfull race, and how advanced they are? they can have space or transdimensional travel but that doesnt mean they are advanced in weapons and other tech like we are (i dont think many space races will have book like we do - Art of war). and lastly what are they motives? Perhaps they travel too long to colonize Earth and only found us here, they dont have resources so invasion is last resort (this is decipted in Defience Series), or they can particular want some Earth resources or they just power hungry race enslaving "lower races and so on this is particulary necessary when you are playing campaing when some scenarios will be enforced by their motives.
As again we don´t need to talk about human miniatures because as always you will use some of manufactures for particular timeline. but aliens is different beast - you can have any that you can want - typical Grey Reticulans, but even some more exotic aliens like Kra´vak, Crusties or some Micropanzer aliens there are lots of aliens and you could check blog Dropship horizon for more info how and where to get them.
About 11 years ago my friend which is game designer came up with idea which he called reversed invasion. The game was never released but basicly was top down fighter-shooter but that doesnt play role here. The game backstory was about that hightech humanity are invading planet of less devoleped aliens, they want their planet,resources and their lives, beign humans that evil but propaganda doesnt say you are bad right?
I think Alien Invasion theme is what i want see in gaming - i have played two such games (using Games-workshop and Forge world Tau Battlesuits/Stealthsuits againts modern day americans in 20mm) first played on sandbox based in Afghanistan and second one was TW demo (1st Edition) based in some European city - what is great about facing hightech foe is atmosphere if you are playing as human one wrong move and you are dust, be prepeared to be beaten many times - but when you achive just one victory you will se how it raise your spirit. It is shame such games are seen more often.
I hope another part of Themes series will help you are get few ideas for gaming any comments of suggestions i will welcome.
Note i found two photos of sandbox game enjoy although you will propably see nothin