Mankind will terra-form Mars in the future, but will do so without nuclear weapons, importing gases, or creating an artificial magnetosphere. We will accomplish terra-forming by activating the internal core within Mars so that convection currents are formed when the core turns against the outer mantle. Aside from creating a magnetosphere, this will generate volcanic and possibly tectonic activity. Tectonic activity has been said to be essential for life, as it brings up numerous nutrients, water, carbon dioxide, and elements necessary to thicken the atmosphere.
A jump start may be all that we need. By building a superconducting magnet on phobos, powerful enough to whip the core into motion, convection processes will begin to churn and keep the magnetosphere running for a long time. Volcanic emissions will bring lakes, oceans, clouds, and possibly even create plate tectonics.
This is why we should wait to colonize Mars. This process, if feasible or it becomes feasible, when completed after colonization would create an inhospitable world for colonists. With talk of 10 000, or even one million potential colonists, evacuation might be impossible.