Every lunar mission passed through the Van Allen belts.
As far as the radiation on the surface of mars is concerned, its comparable to what astronauts experience on the space station. (aside from spikes caused by solar flares) A thicker atmosphere would bring those levels down even more.
Generating a magnetosphere would be the holy grail of making mars truly habitable but I suppose it could be done on a smaller scale to protect human colonies.
They were not exposed to long term radiation that a mission to mars would be. Also have you checked out the trajectory that you would need to have in order to RETURN from mars? You are sitting for 2.5 year on the red planet (of course there would be no way to supply you for that long) before the planets orbit is within reach of the earth again.