What if Earth sends a fleet to Mars with orbital bombardment capabilities? Mars isn't like the Earth where you can just run outside of your fixed settlements into the forests or jungle. Even Earth isn't like that any more.
Also, sending a nuke could be done much faster than an asteroid intercept mission. It could get there in as little as a week.
Just getting the ships in position to shift their orbits will take weeks to months. Meanwhile, nukes already hit. You'd only need 100km/s of delta-v, which is very doable at those timescales with a sub-ton payload.
TLDR: Short of planet killing levels of power, you are unlikely to dislodge dug-in ground forces. Rock is really really good at absorbing impacts.