Humanity has encountered many galactic species. Wars have been few, but mistrust is rife.
Politically, the Alliance is a peaceful trade partner of the TURIANS. As a practical matter, however, there is simmering antagonism and bigotry between both populations over the First Contact War of 2157.
Though humans know better than to unconditionally trust the SALARIANS, their shared restless, reckless ways make them natural allies against the conservative turians and asari.
The KROGAN have no unified government, but individuals are generally treated as potential criminals, a reputation most krogan enjoy living down to.
The Alliance has no formal contact with the QUARIANS. Their Migrant Fleet has not yet passed through any human-settled system.
The BATARIANS are rivals for control of Skyllian Verge. They severed their treaties with the Citadel to prosecute a colonial conflict against the Alliance. Officially, there is no war, but neither is there any peace.