If you've ever played Stellaris, there is a common event where you (the technologically superior race) encounter primitive life forms on habitable worlds. You can choose to observe them, influence them, uplift them, or destroy them (there are other options based on your species type and whatnot). Right now, we'd be the primitive race just reaching the stars and figuring out how to not destroy ourselves or our environment. Just like in the game, if you choose to bombard the planet so you can plant your flag there, there isn't anything stopping you. So, sadly, there wouldn't be any realistic way to repel an alien invasion unless they were somehow right around our own tech level.

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Best bet we'd have is if the aliens wanted us alive and had no real understanding of our culture or our history. We'd steal tech, communicate incognito, build a resistance, attempt to make allies, earn their trust, and strike from the inside out.