"Halo: Holy War" tells the story of the war between humans and the Covenant Alliance finally coming to an end after 2600, and they unite to fight against the Zerg. It's a bit inconsistent with the previous novels, this one is original (a novel based on Halo).
This is a reference to the online timeline. I apologize if there are any major inaccuracies or mistakes. It is not written to follow the main plot of Halo.