Assumption: Intelligent aliens exist, and they match or outperform our intelligence. They also have a technological head start
Question: would the aliens be able to launch a successful invasion of Earth?
I don't know, depends what 'space, and space travel 'is.
I sure don't know.
Sure I know what space 'is, and spaceships, but our current understanding and application, is primitive I suspect.
Including our theories on what we 'should be doing.
Technological development is often surprising and unexpected, to the layman, I assume.
Question maybe similar to Aztecs asking if some unknown culture across the great ocean, could successfully invade their empire.
Assuming they didn't know how 'large the Earth was.
Of course if technology can still be assumed to have room for improvement, A theoretical enemy 'advanced enough, of course 'could defeat an inferiorly advanced foe.
Real question isn't whether it 'could happen,
But whether the conditions 'required for such exist.
I 'think.