In the distant future, Mars may not necessarily afford protection either.

It's going to be sad if the doctrine of mutually-assured destruction some day applies within a multi-planetary context.

Most likely this would take the form of national territories being established on Mars by existing nuclear powers, the weapons either being imported or fabricated on-site.

Of course, the wierder scenario is one where advancements in propulsion technology allow for faster and cheaper delivery of payloads to Mars, resulting in some sort of Earth-to-Mars nuclear strike capability. Transit may remain quite latent, but it could still do the job. Imagine having weeks or months to contemplate an incoming nuclear attack.