Planets are spaceships. Mars is a dead planet either because its shields[0][1] were disabled, or it never had shields to begin with.

So the first thing we'd want to do is activate a magnetosphere on Mars. I rarely see anyone talking about that[2].

[0] https://en.wikipedia.org/wiki/Van_Allen_radiation_belt

[1] https://www.nasa.gov/science-research/heliophysics/real-mart...

[2] https://astrobiology.nasa.gov/news/how-to-give-mars-an-atmos...