You all know how I love questions like this. So here's my reply, in slightly edited form to every gun nut and comnspiracy freak awaiting by their ham radio sets with itchy trigger fingers...
Little green men, grays and hybrids fall pretty esily to most varmint rounds... but before you go out and set up a shooting blind in a hollow cow and await mutilation-happy visitors from space you must keep in mind...
Aliens have the technology to fly through deep space but like any travelers they sometimes forget things like:
1. antibiotics against common microibes and bugs in our atmosphere (War of the Worlds)
2. raincoats (Signs)
3. Directions to the white house
And on top of that...
The fact that if you are gonna invade earth you might wanna try a place slightly less well armed than the rural united states, say downtown London. Just be aware that a soccer thug with three beers in his gut is about as rowdy as a red-neck weilding a shotgun on a saturday night when the moon is full and feeling the need to mate with his cousin. London survived the Blitz and they will survive you.
The goverment of France of course, will surrender and do your bidding. If they aren't on strike. (Just trust me on this one, aliens would be lucky to find a Frenchman who knew who was in charge, and IF he did know he'd harangue the alien menace for an hour about how the current administration 'were a bunch of cultureless mongrels who did nothing for France.' Imagine 6 very confused grey commandos from the mothership going from building to building in vain trying to find the right person to sign the surrender documents in triplicate... meawhile every coffee swilling wanna be artist in Monmarte is joining the resisnatce in the catacombs.)
You can't invade Russia. (Napoleon, Enemy at the Gates, Stalingrad) Unless you do it from the east and your name is Genghis, and you better not be played by John Wayne, that's just silly.
Japan is protected by a variety of giant reptiles and bugs (Gozdilla, Gamera, Rodan, Mothra) and even if you bring your own monsters (Giderah) you will be beaten by a stowaway kid in too-short pants. they also seem to have a vast array of tanks and planes no matter how many times your monsters approach their shores.
China has produced an entire country that does Tai Chi Chuan for fun every morning. According to Danny Glover,(1/2 of Lethal Weapon) "That's some heavy killer ****. Before you poke an old man with a phased plasma array you better be sure he ain't Jackie Chan's sparring coach. (Every Jackie Chan movie ever made). Because even if you kill the old man, Jackie Chan will find you and break your neck like a chicken bone.
Chuck Norris and The Governator live on planet earth, along with Mr. T and the rest of the A-team.
All of our major cities are protected by superheroes. Even Superman is back.
Politics aside, the current president of The United States actually WAS a fighter pilot.(and youy saw how ID4 ended right?)
Rowdy Roddy Piper will uncover your plan, and convince Keith David to help kick alien butt, as soon as he's done chewing bubblegum. And he's all out of bubblegum.(They Live)
Slim Whitman is still alive. (Mars Attacks)
Every mjor American city now contains elite units of Roller Derby girls (do I have to explain this one? Those aliens roll into Sonic for a cherry lime aid and six tough bitches on wheels go Red Dawn on their ass.)
And lastly... there are people, like me who live for JUST such a day... they hoarde food, ammuntion beer, water, flares, jerky, pantyhose, scotch, chainsaws, gas, batteries, Alpo, condoms, and cannon fuse for JUST such a day. They look forward to selecting arms, ammunition, covering tactics, ballistics and wounding characteristics. They train at high dollar fighting school teaching stuff like "monkey fisted peach picking" groundfighting techniques designed to make a bull rhino squeal like a piglet on fire.
I eagerly await your arrival on our little blue planet so my friends and I can discuss the effectiveness of 9mm vs. .45 bullets vaporizing your craniums and whether .223 or .308 with blow your spleen (or phased gloodernoid) completely out of your 4 foot tall frame. We will point out that while you will crumple to a round of birdshot, we prefer buckshot, becuase there is a better chance of penetrating completely through your thin bodies and retaining killing power into the second rank of you little interstellar butt-probers.
We will pass out recipies on cooking your otherworldly flesh and compare it to javalina or chocolate encrusted crickets.
We will line the walls of our homes with your decapitated heads.
Invasion? Bring it on!
We will use your bones for tools.
