If the battle has become so desperate then I'd say that they are getting pounded by Star Wars. And to activate Halo they'd need to go to the ark, and only the forerunners and the UNSC know of it. Maybe more if in the mass of war and an army of that size the Ark's info was given around. And that the UNSC even survive. And if Star Wars are winnign so badly then I'd assume they have some good intel on enemy locations such as the ark.