Well, chances are, if E.T. invades Earth, I'll kill one with my hunting rifle. And if I kill one with my hunting rifle, I'll steal his laser gun. And chances are, if I steal his laser gun, I'll modify it to shoot laser C4. And if I modify it to shoot laser C4, I'll probably go Juggernaught on all of the E.T.'s. And if I go Juggernaught on the E.T.'s, chances are they'll be fearing my insanity. And when they fear my insanity, they will leave our planet never to come back.
And when they leave and never come back, chances are that I'll take a rocket to their home planet and commit genocide.
Conclusion: You don't need to worry about E.T.'s anymore.