In the office of Supreme Chancellor Palpatine's office, there is a relief. How the Jedi didn't notice or suspect this I don't know, but it's a relief of the Great Hyperspace War.
The Great Hyperspace War was the first confrontation between the Sith and the Jedi. It happened 5000 BBY when two hyperspace navigators found themselves on Korriban (hence the "Hyperspace"). Korriban is the homeworld of the Sith species
After the two navigators were captured, the leader of the Sith, Naga Sadow, learned of the existence of the Republic and the Jedi. He decided to attack and conquer this Republic.
The Sith caught the Republic completely by surprise. The Jedi were faced with an enemy they had never faced before, an entire civilization built on the dark side.
At one point the Sith even captured Coruscant.
The Sith captured Coruscant and built a Sith temple.
The Sith's victory was not to last though. They were defeated and nearly driven to extinction. This was the Sith's and greatest first loss to the Jedi but not the last. The Sith as a species would not return for another 1500 years.
I think Palpatine kept the relief of this war to remind him of the Sith's greatest defeat. They had been nearly destroyed by the Jedi and had never been able to have their proper revenge. The relief was a reminder of what Palpatine was fighting for and why the Sith hated the Jedi. The Jedi had humiliated the Sith and crushed them. Time and time again, the Jedi had killed and defeated the Sith. The whole grand plan was so that the Sith could have their revenge and accomplish what Naga Sadow could not, the annihilation of the Jedi and ruling the galaxy. It was the revenge of the Sith, 5000 years in the making.
