Remember though, Space Marines don't win wars by themselves. They are troops that create a landing zone for the Imperial Guard to come down and beat the snot out of the hereteks.
Marines are shock troops. They would have a hard time taking a world and keeping it by themselves without support.
Many thousands of years ago after the Horus Heresy Marine chapters were broken down into 1000 man chapters specifically to avoid allowing them that kind of power.
They could put one hell of a dent in a world, but most likely they would eventually be overwhelmed by numbers.