Now, hear me out here but have you considered Venus?
Cons:
Runaway greenhouse effect, sulfuric acid clouds, crazy hot, etc
Very long days (1 year = 1.92 Venus days)
Nearer to sun => better solar energy collection
Nearly the same gravity as Earth (0.8-ish)
Lower delta-v to get there from Earth (someone double check my math on that)
Once you've got it cooled down, you're a lot of the way there. Bleed off some of that atmosphere (hey, maybe Mars could use it?), get some plants converting CO2 into O2.
It's ludicrous, I know, but I find most conversations about terraforming are. The only real advantage that Mars has is that we could live there while we work on it. Venus, not so much.
It's not clear whether Venus or Mars is an easier target for colonization--that's just a sign of how hard it would be to build a colony on anything that's not Earth.
Actually, despite my above pro-Venus talk, Mars is still an easier option. I wound up on a wiki-spiral just now reading about options for Venus- turns out plants would not work for converting that CO2. The process for CO2 conversion needs Hydrogen- and Venus has none.
We'd need some kind of trade-triangle between Mars, Venus and Jupiter, dumping CO2 from Venus to Mars, then moving H from Jupiter to Venus. We need to find some product Mars has that Jupiter wants, then we'd have it figured out.
We don't really know how long terraforming Mars would take, but it's conceivable that it could take as few as a few centuries, especially given the advance of tech.
https://en.wikipedia.org/wiki/Terraforming_of_Mars#Protectin...
(some scientists say it could be done with current tech)
On the grandest scale, we could take CO2 from Venus and hydrogen from Saturn or Jupiter. The logistics aren't exactly straightforward, but the quantities available are immense.
The idea is that shielding Mars from the solar wind would allow out-gassing to rebuild the Martian atmosphere.
This is slightly tangential, but I think it's important to point out that Mars is more than a survival shelter.
The point of colonizing Mars is to preserve human civilization, not merely our species. Civilization is a higher level emergent thing that can only exist in large prosperous groups of humans. Almost nothing could actually kill the human species, but tons of stuff could end civilization. A self sustaining Mars guarantees our continued social and technological advance.
I don't think it would be nuclear though. Nuclear bombs are delicate devices, the only reason they work on Earth is because they hit too fast to intercept.
Interplanetary war will have at least 1 month long trips between the two planets unless we see some insane tech breakthroughs. A war fleet would be pretty easy to spot and blast out of the sky, as would any missiles launched planet to planet. Hitting something at the insane speeds of ICBMs becomes much easier when you have days to aim.
Worse, the attacker may feel they can use something like that while shooting down any responses.
What else needs a complete overhaul?
My take is that we should go out there, exploiting resources as needed, yet adapting to the environment through technological means. Even if we could terraform Mars, we would still have to adapt in order to go beyond that planet since there is nowhere else within reach that is remotely similar to either planet. Even if you could imagine a sci-fi future where the stars are within reach, chances are that a good portion of the habitable planets will have life of some form. That would raise more serious ethical issues than the subjectivity of beauty.