For a more realistic take on planetary defense, there's always Ender's Game/Ender's Shadow. They actually realize that they can't protect themselves in all directions in space, so they launch everything to try to end the Buggers first. And even then, I'm pretty sure they have small fighter squadrons trained around Earth in case they're attacked in the meantime.
A main thrust of the plot (at least of the original short story) is about the vulnerability of planets - it's worth a read if you've not come across it.
The humans have the Dr Device that can destroy anything it can get close enough to, but they know they will be hugely outnumbered when approaching the Buggers' home planet - and hence need a tactical genius who can overcome that numerical disadvantage. Most of the story is about how that genius is developed to its full potential.
(Why didnt the Giant Eagles just fly the Ring to Mt Doom?)
They needed to stay out of the gaze of the giant eye to prevent their detection. Nothing would prevent the orc army from shooting the eagles from the sky having the hobbits plunge to their death in the process.
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A main thrust of the plot (at least of the original short story) is about the vulnerability of planets - it's worth a read if you've not come across it.
There's a solution for that!