This is a little depressing, but I just realized if we ever have interplanetary war, we'll just push big space rocks into their orbit.
https://en.wikipedia.org/wiki/The_Moon_Is_a_Harsh_Mistress
The moon colony uses this to win its freedom.
However, The Expanse is also a great book
For the first few seasons of the show I thought they did a good job, even though (or maybe because?) the show departs from the books in a lot of ways. But they tried to cram way too much into the last season and just made it seem like jibberish.
Example of it being different process: bullets are accelerated by gun powder, but stopped by various armor.
Example of increased dificulty: interception rockets have higher demands on speed and agility. (If somebody tries to evade, you have to be faster)
The problem of intercepting an asteroid heading towards you is exactly the same problem as intercepting an asteroid heading along any other orbit. Again, if you can redirect an asteroid, you can redirect an asteroid.
A more apt analogy would be a sword fight, where the counter to someone swinging their sword is swinging your own sword to redirect it.
Important limitation can be earlyness of detection, and equipment delivery/deployment speed, and redirection rate/speed.
Attacker could work with slow equipment deployment and redirection rate (maybe take years), but defender will need certain ratios between detection esrlyness and speedy deployment of equipment to start the deflection.
And if I’m mr Evil, I would pass the commet through some gravity assists to increase the speed - to bring the example closer to the bullet case.
The delta-v required for a heavy asteroid may make it more practical to send a fleet of nukes though.