in the movie,
the flood take over the planet by the gravemind, and the foruners stay behind sending pods filled with each species of life, humans, elite, grunts etc. to dif planets.
Playing defensively is possible, but attempting to defend every direction is incredibly hard, and you'll likely be minced by the enemy as they
take over the
planet and use the extra resources to simply overwhelm you with a
flood of units.