If at the end of Halo 2, Cortana decides to detonate In Amber Clad’s engines it would basically not only end the war but the series. Think about it.
If In Amber Clad blows up > High Charity blows up > Keyship blows up > Truth/Tartarus is killed > Delta Halo blows up > Flood is destroyed > Covenant is destroyed > Delta Halo doesn’t go into standby and Humanity doesn’t have to go to the Ark > Chief never gets stranded in space for four years > Requiem is never found by Chief > Didact is never awakened by Chief (and if he is in a controlled environment by Infinity not one ignorant Spartan) > Cortana never dies > Guardians never awakened > Galaxy fixed.