See the difference between the UNSC and the Imperium, is that if the Imperium start losing on the ground, they do the same thing. Their ships execute Exterminatus or Virusbomb the planet and destroy every living thing on the surface. The Imperials also have planetary defenses to stop other factions from doing exactly the same thing, so just imagine how different the war in Halo would have gone if the UNSC had ships and defense systems to deal with that.