I mean I know in the Halo games that the UNSC would have lost but if they did things differently could have they won. I mean they could have adapted faster and made better strategist early in the war. They could have also used slip space bombs or Biological warfare. Near the end of the war were winning in the kill to death ratio. I mean look at the Pillar of Autumn it destroyed 4 Covenant Battlecruisers. And when they invaded Reach we lost 130 ships while they lost over 262 ships. Think if we built Orbital Defense Platforms on all are planets. We definitely had the time to build them. We also could have used forerunner artifacts to lure them into traps, similar to what Preston Cole did in the Battle of Psi Serpentis, the UNSC lost 160 ships while the Covenant lost over 300. You can also look at the Battle of Sigma Octanus IV it was a loss material for the UNSC but we destroyed 16 Covenant ships without losing a single ship.