There is, however, a scenario that gives us a proper alien invasion we can fight against, even win, though there are darker implications.
The first wave of invaders catches Earth unawares. We are pressed hard but somehow, fortuitously, the aliens can be engaged and defeated by our weapons. They drop rocks on a few large targets and then deploy troops with a large fleet of landing ships, fighting on land. This is the Martian scenario straight out of HG Wells but our tech is sufficient to fight them to defeat. They may have higher technology but we have them outnumbered and, by Jove, our fighting spirit is second to none!
We win, pat ourselves on the back, and start rebuilding while our scientists reverse engineer the alien technology.
And here comes a second wave of invaders, somehow spaced just far enough apart for us to have created a united planetary defense force, integrate the new weapons technology, and bring hundreds of divisions online. Our blaster tanks are supplemented with mobile infantry suits and antimatter bombs and other scifi pew-pew.
We win. And there's another interbellum period. Research, refit, rearm. We're now deploying space forces and can confront the enemy on the high frontier. Another invasion.
We won. And now we're adapting alien bioengineering technology to change ourselves, to create human warriors. We're accelerating AI research to create the first conscious machines. And humanity will triumph again! The propaganda reassures us of how wonderful we are, how perfect, how we represent such a threat to the universe that aliens cross the stars to confront us.
Does anything about this strike you as artificial? Certainly it does to some of the top military theorists. The threats are enough to challenge us but never quite destroy us. It is a selection pressure and we are changing under its influence. These alien threats fit neatly into our conventional science fiction fears, as if ripped from our subconscious. These aliens appear to be constructs. And where are the true aliens? What are they? Is the vast, inhuman intelligence sitting out in the Oort cloud, fabricating the next threat? Our world is a Petri dish, we are the culture, the unknown intelligence the experimenter. What are they shaping us into and why?
And the alarm sounds. The next invasion has begun. To the ramparts! To the ramparts!