According to the prehistory timeline on Wikia,
1003 B1: As a young girl, Ymir Fritz gains the power of the Titans from an unknown source, becoming the Founding Titan. With her powers, she is able to turn her subjects into Pure Titans.
The Fritz family comes into power as the royal bloodline of Eldia, destined to remain unbroken for the next 2,000 years.
990 B1: For unknown reasons, Ymir Fritz dies 13 years after inheriting her power. Her "soul" is split between Nine Titans that are inherited by nine Subjects of Ymir, with the Founding Titan belonging to the Fritz family and the eight remaining Titans leading clans subservient to the Founding Titan. Those who inherit the power of the Titans are destined to die after 13 years as Ymir did due to a phenomena called the "Curse of Ymir."
The Nine Titans wage war against the nation of Marley, the dominant nation of the land, taking control of the continent and establishing the empire of Eldia. Eldia conquers countless other civilizations as well using Pure Titans as cheap weapons of mass destruction. This results in massive casualties and entire cultures being wiped out.
Unknown: Eventually, in-fighting breaks out between the various Eldian families for control over the eight Titans, but the Fritz family, using the power of the Founding Titan, maintains balance.
The Ackerman family, an Eldian bloodline resulted from the Eldian Empire's fiddling with the Subjects of Ymir, becomes the sword and shield of the royal family, remaining close to Eldia's king for generations.
Eldia becomes allies with Hizuru.
When the Titans arrived on Earth, what year is it?