Is it possible to start 'terraforming' venus, by engeneering plant cells to float around in the upper atmosphere and convert CO2 into O2 over millions of years to bring down temperatures? Obviously they would need Genetic Engineering to survive the acidic upper atmosphere.
Mars, but the best you can hope for is a something where it is like living on top of Mt Everest.
Building O'neil Cylinder Colonies at Lagrange points is a better use of time and resources.
you need to harness much more energy than you can in nuclear weapons. Take a dwarf planet like Makemake or Ceres and change its orbit to crash into Venus at the right angle and velocity. So that they both reform into a planet and moon system. Then pray it is spinning and forms a magnetosphere. Then you can start crashing water ice asteroids and comets into it.
This is also going to take millions of years.