Born Under Mars — John Brunner

When mankind colonized the stars, they travelled out from Earth in two directions - to Centaurus and its Southern Hemisphere neighbours and to Ursa Major and the constellations around Polaris. And strange to say the humans who settled on those various worlds began to develop into two differing antagonistic types.

For Ray Mallin, born under the surface of Mars in the sparse colony of Earth's inhospitable old neighbour, neither the anarchic 'bears' nor the autocratic 'Centaurs' commanded his loyalty. So when secret agents of both galactic groupings suddenly focus their unwelcome attention on his most recent star-piloting mission, he knew only that something of vast significance was up - and that he unknowingly was the key to it.