We'll have to put something on Mars, simply because we will need the mineral/ore resources from the asteroid belt, in order to establish deep space travel. Terraforming could prove to be difficult though, simply because we will probably need a couple of nobel prizes in both magnetics and superconducters in order to reset that magnetic field, which will be necessary if we want an atmosphere with a lifespan greater than a thousand years or so. I believe that is the only real hurdle.