Crossposting from the trope page for reference:
Reconcile The Bitter Foes
: Shepard has a chance to end the war between the quarians and the geth
peacefully, though there are many conditions that have to be met for this to succeed. Also, the Krogan/Salarian/Turian enmity can be mended to the point where the three races start cooperating—something deemed impossible for at least several centuries. The Turians and the Humans have the latest in species grudges, but the Turians are the most willing to help humanity out (though they have their own problems).