[JP]: this implies aliens are more advanced than human. I think the fact that they traveled to Earth implies that they're more advanced than humans.
[HST]: but what makes you assume the aliens would be the stronger civilization? Relative
strength is a more open question. The alien civilization can probably kick ass back home in their home system, assuming they didn't nearly destroy themselves in wars. But in the actual universe as we currently understand it, with no magic hyperdrive capabilities, there are going to be very severe limitations on how much stuff you can transport across interstellar distances. So it's quite possible that the even though the alien civ is very powerful and advanced, its power on Earth is not so great.
A lot depends on what the aliens, want, and what they can achieve. Do we have any resources which are valuable to them which they can actually make use of? E.g. maybe they want living space, and our planet looks good to them because their living requirments match ours to a significant degree. (Fat chance, but maybe.) Still - if they don't have the ability to transport a large number of their population here
cheaply, this isn't really going to do them any good. There's an excellent chance that any physical resources we have just aren't going to be worth the effort for the aliens to try to acquire.
There's a big range of possibilities. Best-case scenario - aliens have no practical interest in acquiring any physical resources we have. Maybe they just like gathering information, and want to learn about us. Maybe even though they think our technology sucks, they think Mozart and Shakespeare are pretty cool, and so they want to
exchange info with us, and they send over a tiny cultural exchange embassy of some sort. Humans could be OK in this situation. Though there would be massve effects on the culture I think, unless the existence of the aliens were kept secret. We'd probably have a bunch of new religions in California based around the newcomers... waitaminnit, we already have that.
Worst-case scenario - we have something that the aliens want to acquire, and they have no scruples about taking it the hard way. Or maybe we just piss them off by firing a missile at their ambassador, and they figure things will be much easier for them if they remove the infestation of carbon-based life from the third planet before they move on with their plans to colonize Jupiter's moons or whatever. They could expend a relatively small amount of energy altering the orbit of a decent-sized asteriod or comet, then wait a few years and let it impact earth and cause massive environmental disruption. Meanwhile set up self-replicating machines to harvest resources from the asteroids and build up an army of killer robots.
It may take a bit of time, but if the aliens really
want to wipe us out, our chances aren't so good unless we detect them early on and recognize the threat.
Well, there are lots of scenarios in between. For an extensive catalogue of possibilities, vist the SF section of your local bookstore.