The Birth of the High Republic Era
Star Wars: The High Republic has been a huge success over the last three years, and it's possible a Sith failure is responsible for the entire era. Just over three years ago now, Lucasfilm introduced the High Republic Era - a time of unprecedented scientific advancement and growth, as the Jedi and the Republic advanced into the Outer Rim. They did not go unopposed, however, and already the story of the High Republic is revealing enemies who exposed the weaknesses of the Jedi - and potentially inspired aspects of the Sith masterplan.
The High Republic era Jedi
The High Republic Era took place from 500 to 100 BBY, meaning it began 468 years before the events of Star Wars: Episode I - The Phantom Menace. No doubt the flourishing of the Jedi and the Republic was much to the chagrin of the Jedi's greatest enemy, the Sith. However, the Sith may have been responsible for it - through their own failures.
The Reformation and Advancements
The story of the 'modern' Republic and Jedi really begins around 1,000 years before the Battle of Yavin, after the defeat of the Sith. With the Sith believed extinct, the Republic began to heal and recover from the Jedi-Sith War. However, something changed 500 years later in 500 BBY that led to the greatest era in both Republic and Jedi history. Clearly, something big happened in the first year of the High Republic Era that launched this Golden Age. There is evidence that 500 BBY was a time of reformation in the Jedi and the Republic, with Huyang name-dropping 500 BBY as a key date for his archives of lightsaber construction.
The High Republic Era was one of exploration and growth. According to the Star Wars: The High Republic Character Encyclopedia, discoveries in midi-chlorians led to a fresh understanding of the Force - setting up the creation of technology to detect midi-chlorian counts by the time of The Phantom Menace. Advancements made during the High Republic Era changed the Jedi forever. Scientific advancements in the area of hyperspace navigation also helped the Republic, making it possible to expand into the Outer Rim.
The Influence of Sith Failures
Interestingly, Star Wars Legends may hint at what happened in 500 BBY to cause this reformation. In Legends, 500 BBY was the time of a secessionist crisis involving one of the Core Worlds, Corellia. The people of Corellia sided with Selonia and Drall in a war to gain control of the Federation of the Double Worlds, the government that ruled the planets Talus and Tralus. While they lost this war, they tried to secede from the Republic 30 years later. This did not go well for them. They were able to temporarily close their borders and withdraw from the Republic, but ultimately they were forced back into the Republic.
Corellian Shipyards in Ahsoka.
While the Jedi and Republic didn't know it, the Sith remained active during the High Republic Era, bound by the Sith Rule of Two and working in secret to undermine their enemies. It is entirely possible the Sith could have been influencing the secessionist movements, foreshadowing their later role during the Clone Wars. If that were the case, they may have wanted to take over, or, at the very least, make sure the Republic didn't have it. However, this backfired and their failure to keep Corellia could've led to the High Republic Era.
Sad Darth Vader and other Sith Lords