So it's everyone's favorite bad guys from the Starcraft universe vs everyone's favorite bad guys from the Halo verse (or second favorite, if you preferred the Flood).
Basically,the Zerg (led by Kerrigan at the height of her power) attack the Covenant at the height of their power (presumably before Master Chief blew half their fleet up).
It's a war of total destruction. The Halo rings cannot be used by either side to achieve victory, which means both sides are going to have to do this the hard way. Who wins.