Since Treaties have been historically broken, they are completely meaningless. At first I think the people that are able to reach Mars will be governed by the people that enabled the move. The government of the private company.
When the colonist have reached self sustainability then the people will find away to govern themselves. It will be difficult as the People of Mars would be the smaller population of Earth/Mars conflict. We have seen throughout history that we as people are willing to commit all kinds of atrocities in the name of peace. Just because we would now be on two planets will not negate this human habit of war in the name of peace.