The Boss Cass War was a tragic and destructive conflict that resulted in, possibly, the highest number of deaths ever recorded in Earth's history, aside from the War of Deceit, the Mythic Dawn War, and the Age of Chaos. It began when the political extremist Boss Cass, under direction of Malefor, unleashed his armies across the world and seized whole countries, adding them to his empire. The conflict involved Malefor's Holy Empire, the UNSC, and whatever other militias arose.
Even to this day, the world has not forgotten the war. Every now and then, people will discover old bunkers belonging to the Empire, voice transmissions, statues of Boss Cass, and typically any mention or imagery of the dictator will create discomfort and outrage amongst people.