It's silly to consider nuclear bombardment of Mars when your objective is to build permanent self sustaining settlements there. The big problem is that Mars lack of gravity is believed to be the reason it became such a hostile environment, if it was every truly habitable at all. This could only occur if the core is significantly less active today than it was in the past. The first challenge is getting people to Mars alive and overcoming the obstacles to survive and prosper on Mars as it exists today. The next would be redirecting the mass of asteroids and comets being pulled into the solar system on course for Jupiter, towards Mars, Phobos, and Deimos to increase their mass. If you increase Mars planetary mass, it will have more insulation to maintain core temperatures. Increase the mass of her moons and they can exert gradually increasing tidal stresses on Mars and in doing so gradually increase the cores activity, thus increasing the planets gravity, reviving the Martian magnetosphere.