Nameless marks his grave

War comes to the UNSC, and though neither side wishes to fight, their principles and pride do not let the fighting come to an end.

The UNSC and the Turians seem to me, like the perfect matchup of unstoppable force and immovable object. Any sufficiently fortified human world would be practically unconquerable to the Turians thanks to the ODPs incredible range and strength, while unshielded human ships would struggle incredibly compared to ME ships, which are both faster and shielded. In conclusion: neither side could decisively defeat the other.

There is going to be no covenant or reapers, and while there will be some forerunner artifacts, they will not play a major role in the story.
Rough timeline:
Forerunners die to the flood - 100 000 BCE
Protheans destroy themselves in a civil war, with the factions remaining afterwards being too small to survive, outside the collectors - 50 000 BCE
Everything afterwards can be taken according to the ME timeline

Disclaimer, while I would like it to be the case, I do not own the rights to Halo or Mass effect, they belong to 343 Industries and EA respectively.