[duckduckgoose]

Thats not entirely true. Have you ever heard about the studies the military has done? Where the more advanced you get it becomes easier for a less advanced opponent to destroy you.
I was reading about it when the military botched one of their war games, basically they pitted high tech equipment against a rebel force that was using much less advanced stuff, they expected a landslide victory from the high tech devices only what they got was utter defeat and humiliation when their precious technology failed them.

Anyway point is we're pretty well defended, yes a alien race that is advanced enough to visit our planet could be packing some mean weapons, but that doesn't mean that they could take this planet down easily. We have nuclear weapons capable of space flight (I'd like to see something try to shrug off a nuclear blast in 0 gravity, even if by some miracle they had armor that could protect their ship from being destroyed theres still the radiation and the massive shockwave that would rattle everything inside their ship around like a pinball machine), and our ground troop weapons would kill a organic creature from another planet just as easily as they kill humans.

Edit: I just wanted to expand a bit on some things, the magical armor I was talking about that could block a nuclear blast does not exist, the universe is made up of the same material and there is no known material that is strong enough and heat resistant enough to actually hold up under a nuclear blast, the heat from the blast alone would flash melt anything in its range.
Another reason why we wouldn't be in to great of danger is because of our nature, we would not have a single problem with using nuclear weapons, thousands upon thousands to save our planet. We are by nature a hate filled and vengeful race we will use any weapon to protect ourselves and destroy our enemy, if some alien race was to suddenly appear and attack us and we started losing we would most likely nuke ourselves into obliteration just so they couldn't have the planet, kinda like a final **** you to them because thats just the way humans are.