When someone contacts Quarians about old machinery, they should think twice before engaging with an other-wordly yet so familiar species, a war, and circumstances that will shock two galaxies now that the rift is open.
Before the Protheans or Forerunners, there was a war between Titans. That war is still ongoing. In the face of such forces, what could the UNSC, the Citadel Allience or the Covenant do; especially when they are embroiled in their own wars with one another?
After the destruction of Requiem and an attack against Earth, both the UNSC and Covenant Separatist form the Systems Alliance. Then a discovery in the Deca-5 system changes everything for all sides. Cold War like conflict, lots of ONI , AU for Mass Effect and a council that actually thinks before it acts. Awaiting its sequel.