There are nuclear weapons in China Iran Korea and Pakistan. It wouldn't take much to send a couple of warheads off on this planet somewhere that would cause a lot of environmental damage then if you have got someone who wants to retaliate you have real problems.
Perhaps the best motivation for going to Mars is political. It is obvious that no single nation currently has either the will or the resources to do it alone but a consortium of nations and space agencies could achieve it within 20 years.
