As you may know, Mars, despite being the best candidate for colonization in our solar system, is pretty inhospitable. In comparison to Earth, it has very little mass, (only about 11%) which means very little gravity.
The low gravity also means that little atmosphere clings to the planet, essentially none of which is oxygen. Also, there is essentially no water that we know of.
So what are we to do? Terraform! All we have to do is crash an almost endless procession of asteroids into the planet, adding mass and water. After doing that for a century, we could start atmosphere 'factories' designed to change the crappy atmosphere of Mars into a nice Oxygen/Nitrogen affair.
There are simpler ways to terraform a planet, and it would be possible to live there today if we were willing to live under domes. But why settle for less?