Conquest of Rhûn

The Conquest of Rhûn was the last event of the First Easterling War and the first expansion of Gondor's power.

History
After King Turambar defeated the Easterlings on the field in TA 541 avenging his father's death, Turambar led Gondor's armies deep into Rhûn past the Sea of Rhûn. All the lands east of Dagorlad, south of the realm of Rhovanion, north of the Ash Mountains, and west of the Sea of Rhûn fell to the might of Gondor. The conquest was complete by TA 550 and marked the foundation of the East-lands.

Aftermath
Whether this meant Gondor ruled over Easterlings or subjugated them is not known only that these new eastern territories became a province of the Kingdom of Gondor. It is possible that the Easterling population was expelled from the territory and forced further east where they continued to live. This seems likely since new tribes of Easterlings, organized and still hostile to Gondor would appear though it be over a milennia later.