Before joining the Citadel Council that governs much of the Milky Way, the Turian Hierarchy was a formidable power with numerous client species supporting its powerful military. Rapid expansion from Palaven later led to problems with outlying colonies, when isolation and xenophobia led to infighting in what was termed the Unification War.
The turians made first contact with the Citadel around 700 CE, when the Krogan Rebellions were raging. When the krogan devastated turian colonies, the turians responded by unleashing the salarian-developed genophage against the krogan. Two hundred years later, the turians were invited to join the Citadel Council.
In 2157 CE, human explorers attempting to activate a mass relay were attacked by turian forces, as unauthorized relay activation was strictly illegal. The resulting conflict, the First Contact War, saw the human colony of Shanxi occupied by the turians before the Citadel Council negotiated a peace.