Terria has always been a diverse country. It has always been occupied by both Elementals and Humans, they lived together in harmony. Elementals would use their powers along side Humans and other countries looked at them and scoffed. Yet this peace was challenged once Benders were discovered. Benders haven't always "existed", people have always known of some Elementals' had more connection to their powers than others, but the idea of someone with abilities that not any of the other Elementals have... terrified the public.
Terria wasn't just known for its blended community, it was also a very large, prosperous, and up-and-coming country so many people were emigrating here to build better lives for themselves. While for the most part people are supportive within the country it created a toxic sense of nationality. Which combined with the first EVER documentation of a Bender, the country began to split between the two groups. Humans were attacking Elementals and vice versa. Then another Bender was found and then two more.
These four Benders, one from each element, banded together in an attempt to stop the prejudice. Their powers were extreme, powerful, enough to create natural disasters that no Bender could do on their own now. They waged war on the Humans who were set on belittling Elementals. Hundreds of thousands of humans were killed in the three years the war was waged, and hundreds of elementals went into hiding while even more died in crossfire. When suddenly they vanished.
Many Elementals believe they left once the noticed the damages being brought to their own communities, that they went into hiding to protect themselves and the ones they loved from their own powers. While many Humans believed that they merely ran away or got bored of responsibility that came with leading a rebellion. Yet no one truly knows.
It's almost been a century since the first four Benders left in the middle of the war they started. While Terria isn't back to where it once was in unity, it's much better than during the war. Yet the country has never been more divided. Benders have never been as powerful as the original four, but would that truly help Elementals gain the status they once had?
(Subject to edits)