Assuming you are talking about an Alien invasion, here are some of my favorites I've run in the past.
#1 D'Vorn: They were originally humanoids from an old superworld module, that I had run years ago. I wanted an invasion, so I had them return. I made the whole race refugees from their world, which had been overrun by another group of aliens. They had escaped in a STL cyrogenic ship centuries ago. Of course their computer failed to wake them up, and Earth Astronomers mistook their planet for the moon of Pluto, Charon.
A group of them awoke in the late 1980's, and traveled to Earth to get spare parts to awaken the others. (this was the original Superworld scenario) Of course their ship crashed, and some superheores helped them fix it.
15 years pass. A supervillainous scientist discovers radio signals coming from Charon (where the D'Vorn are still fixing their ship. He travels out there in his private spaceship, and contacts them. Of course he brings along his supervillain crew.
Now the D'Vorn society is a caste based society, with what caste you are in determined by mental, psychological and physical testing. 8 castes existed in total (they had 8 fingers and thumbs) - Scientist, Administrator, Military, Farmer, Labor, Engineer, Artist, and Criminal.
the supervillains of course stage a coup, and put the criminals in charge. Why did the fleeing D'Vorn take the criminals with them into space? By ancient tradition the criminals were useful...they performed the lowest, most menial labor. They were also subjects of medical experiments (so some of them had quasi-powers)
With their criminal allies, the supervillain launches an invasion of the Earth...his target of course being the oil-rich Middle East.
Conflicts the mini-campaign had
~Heroes allying with corrupt middle Eastern dictators, to fight aliens
~Finally reaching the "Home Planet" of the aliens (a giant spaceship), and finding out the true situation, and that the other castes weren't so bad.
~Villains used the opportunity to go on crime sprees in many areas
~The government sent out a modified shuttle, on a suicide mission, with one of PC's in charge, to take out the enemy spaceship. Of course there were also 12 criminals aboard, who were promised pardons
Eventually, the pesky heroes won. The legitimate D'Vorn government (a council consisting of the leaders of all 8 castes) was restored. Since it was so slow, the D'Vorn ship stayed around, turning the campaign into a semi-cyberpunk one.
#2 The Methane Breathers
Remnants of a once great civilization, that consisted of many different races, the Methane Breathers centuries ago started a neo-fascist movement, and killed off all the oxygen breathers in their area of space. They had such a high level of technology, that a single Methane Breather would by himself command a single ship, crewed by hordes of robots.
The Methane Breathers had a very low population, and actually consisted of several different races (One from the Zodiac Conspiracy), all of whom breathed Methane. Their Robots did all the actual work.
Among their other activities, the Methane Breathers needed huge amounts of raw materials to keep their industry going, and sustain their ridiculously high standard of living (I mentioned they each owned their own spaceship?)
In order to sustain this level, they strip mined any world they could find, and it just so happened that Earth had something they wanted.
3 ships (and thus, 3 aliens) assualted the Earth with their legions of robots, bombing everyting in site, to get rid of the infestation of Oxygen Breathers. Many of their attacks were to gather components, to build more robots aboard their factory ships.
The fun part of this one was I had all the real life political leaders in office. Bill Clinton of course cut a deal with the aliens, promising to mine the mineral they wanted, in return for them sparing the Earth. To enforce their claim of tribute, the Methane Breathers left behind their robotic ambassador...Methanon.
Conflicts
~Obviously, this campaign was about racism. They were just biased on what atmosphere you happened to breath, and their were aliens.
~Smashing robot hordes
~visions of Independece Day
#3 EnGulfCo
I ran this around the Enron debacle. A corporation got hold of a "gate" that led to another worl (ala Stargate). Of course the Corporation launched an invasion, hiring a bunch of mercenaries to train a faction of the locals. Once the locals were in power, the corporation traded them more military equipment, in return for everything they could grab. Of course the corporation was an Oil company.
All the locals on the planet originally came from Earth. They lived in a quasi-medieval background, except one area was run by an actual dragon, and they had wizards. All their nobility were metahumans, to give the PC's someone to beat up.
The PC's got introduced to the scenario when the mystic was "summoned" by the wizards to the other world. Thus, they also had to find a way to get back to their world, and help the locals.
Conflicts
~The main function of this one was to show Corporate Greed
~The fact the corporation was introducing literacy, and freeing peasants. The corporation had been running around the planet for about 10 years.
~the Corporation was stopped when a wealthy PC bought controlling interest, via the stock market
~Of course I used maps and background from all those old Fantasy games I have laying around.
~trials of the various executives continue to get mentioned in the background of the campaign. For some reason, the PC's hate these guys more than their supervillains...
As you can tell, I run by invasions a bit different from the comic format. I try and put a little thought into them, and give a reason why the invasion occurs.
Oh, first post.