Yea I had to put some thought into this, and this is what i came up with. Mars has a small atmosphere due to its relatively small size, but its not its size that matters. Millions of years ago Mars lost its magnetic field when its core began to cool due to its small size. The process in which we call plate tectonics was slowed to a halt. this stops the recycling of continents and active volcanoes which release massive amounts of atmospheric materials. So if we were able to drop a couple nukes into mars's core to restart the process and regenerate a strong iron magnetive core much like earth's a stable atmosphere can come back. The other situation is the distance from the sun, but mars is not too far to be unterraformable all you have to do is control the amount of energy emitted from the planet than received. In other words, create a green house effect, we do a pretty good job of melting our glaciers here on earth why not do the same on mars? well mars has no oxygen so our primary pollutant is out of the question but what if you were able to construct massive solar panels to reflect more sunlight onto mars giving it that greenhouse effect and warming it to livable conditions. For the oxygen there is enough frozen water on mars to create enough habital glass domes to start growing trees..photosynthesis. The final question is why go to mars how is it economically sufficient? Well incase you didnt know already mars is coated with iron oxide or rust which gives it it's red color in my opinion iron on the surface is 100 times easier to gather than in the ground. mars can also be a docking point for mining asteriods which can be made up of 100% gold! and other valuable minerals.
If there is any question about our future with space it starts with the moon its pointless to make a base on mars without a base on the moon
I know i wrote alot so im going to be editing this out alot I hope this was helpful in showing you all my ideas :>).