If we attained the technology to colonize Mars, there would be war on Earth. Because humankind hasn't evolved, so as soon as new territory is up for grabs, governments WILL grab, and since they will realize that waging war on Mars is way too expensive, they will fight their enemies on Earth. If they defeat them here, they don't have to fight them there.
Ironic, isn't it? The prospect of life on Mars could bring (more) death to Earth.