In the 1960s the United States government started a project called 'Far Side.' The project is unknown to most of the general public. The idea behind Project Far Side was really quite simple. The government would use a large radio device to broadcast messages into space. The messages broadcast would be simple patterns of energy. The patterns would be classical mathematical patterns that would identify them as being originated by intelligent life. The source of the radio waves could be traced by any civilization capable of receiving and analyzing the signals. The humans of planet Earth would announce to the universe, "We are here!"
Project Far Side is started with a bit of fanfare and then quickly forgotten by most of the world.
One group who don't immediately forget project Far Side are certain statisticians. They use astronomical data and the history of discovery of new civilizations here on planet Earth to project what might happen as a result of project Far Side. The statisticians discover that there will likely be several civilizations that will receive the radio signals. Most of the civilizations won't respond, for reasons that will soon become clear. A few civilizations will respond. The civilizations that will respond will be interested in one objective: the conquest of the Earth.
The statisticians bring their calculations to the attention of the United States government who discuss the matter briefly and then, not only shut down most of Project Far Side, but also start up other projects with exactly the same name to hide the fact that they had ever been stupid enough to start the original Project Far Side in the first place.
I say, 'shut down most of the project.' It is pointed out to the politicians that, although we of planet Earth are no longer broadcasting our greeting to the universe, there are still the messages out there that we have previously broadcast as a result of Project Far Side.
The authorities to 'come up with a solution to the problem' draft my first predecessor. The man, call him Zero, fails to solve the problem. He does, however, succeed in deploying a number of radio telescopes that will detect approaching interstellar vehicles, that is, spaceships. We of planet Earth can't defend ourselves against hostile aliens, but at least they won't take us by surprise!
There have been a few Zeros. Some of them come up with really great defensive ideas. One such idea is the use of death rays on the invaders. It is pointed out to Zero that we don't have any death rays [scratch one brilliant idea.] There are more Zeros and more impractical ideas.
You say, 'We are lucky that no aliens invaded planet Earth!' However, you are wrong. Aliens do invade planet Earth and are defeated. They are defeated by the heroic actions of legendary [if unknown on Earth] Wiking warriors and my defensive plan.
The problem with the first Zeros is that they are heroic, grim jawed military officers who try to think of a military solution.
I am the current Zero. I used to run a strip club that was little more than a front for a whorehouse. I am the Zero who develops the 'Strategy of the Twins.'
The reaches of interstellar space are immense and the first alien spaceship to reach Earth is a scout vessel. No, dummy, it is not piloted by a heroic lone scout! In order to run a spaceship you need an around the clock crew on watch for space debris, nova stars, black holes, things like that. The Geolan ship has a crew of 14. Since the Geolan are warlike, the scout crew is all young, male warriors. They have weapons of enormous power and a science that is millennia ahead of planet Earth. The Earth has only one practical weapon against the Geolan warriors and I deploy that weapon. Of course, the weapon of which I speak is pussy!
The Geolan scout ship follows a naive welcoming beacon down to a remote desert landing. Earth warriors greet them. Wiking warriors to be exact.
The savage Wiking warrior men wear metal helmets, armor to protect the torso, greaves to protect the legs and high boots. The Wiking women wear metal caps and ankle boots only. The Wiking warrior men brandish classified weapons that vaguely resemble laser swords. The Wiking women have only the weapons they are born with.
The Wiking women approach the Geolan ship with a clever message, in English, for the Geolans. The clever message is, "Hiya, big boy!"