Aliens.
Earth isn't the only planet in the universe with life on it, that's just statisticly impossible.
"In 2011 NASA's
Jet Propulsion Laboratory (JPL) and based on observations from the Kepler Mission is that about "1.4 to 2.7 percent" of all
sun-like
stars are expected to have earthlike planets "within the
habitable zones of their stars". This means there are "two billion" of them in our own Milky Way galaxy alone and assuming that all galaxies have a similar number as the Milky Way, in the 50 billion galaxies in the
observable universe there may be as many as a hundred
quintillion."
Source
No, that doesn't mean I believe in little grey humanoids abducting people for anal probing.

But at some point in the future we will find extraterrestrial life, or they will find us. It could happen tomorrow, it could happen in the year 3014, or anywhere in between. If they find us, it means they are more advanced then we are, and if they turn out to be hostile, I doubt that's a war we could win.