It's a really improbable scenario.

Did'nt Ron Reagan say the same thing in a speech early on into his presidency??

I don't believe this is true. We can't even unite to fight common human enemies from the desert wastelands of our own planet. But, at this point in our world's technological development, there is not even one country that could "field" a large-scale interplanetary defense force. And then there's the logistics of interplanetary invasion. We can assume that any invading alien races that have worked out the difficulties of transporting foodstuffs, weaponry, and personnel across interstellar space to our planet have set up support bases along the way, perhaps even within our own solar system, which we also have no way of striking against. And then there's the matter of intelligence gathering. We can assume an alien race that has overcome the difficulties of logistically supporting a planetary invasion has gathered enough information on our languages, cultures, and defensive strengths to know who and what to hit first in the initial attacks. How do we get inside the OODA loop of an alien invasion force? Face it, when the Vorlons come, we're hosed.

i better go buy a towel then :)

The skunkworks will save us.

An alien race could incite violence and war between nations in a large scale like world war II before they come in to attack our weakened selves. Unless they need humans for food?

>Unless they need humans for food?< That is but one way around Fermi's Paradox...