Actually, what Mars needs is something more permanent than the temporary heat from nukes. A moon large enough to sufficiently add to the tidal kneading it receives from the sun should do nicely. Collecting the asteroids in the asteroid belt just might do the trick. ~ JN, Ph7.5
Nukes would be a ridiculous waste of resources when dropping rocks and ice down a gravity well is an option anyway.
A large moon could liquefy the core and grant Mars a magnetosphere. But I’m not sure how effective it would be at warming the surface without significant volcanism.
Space-based mirrors could be used to reflect additional light at the surface or a giant lens (to refract sunlight), increasing the at surface solar energy available.