The Destruction of Telos IV, or the Destruction of Telos, was a statement made by the Mand'alor Empire to the rest of the galaxy not to mess with them. To the Republic and Jedi, it was an act of war and the final straw in the series of events during the year 3,730 BBY.
Telos was destroyed in spite of the victory at the Rata Nebula by the Chiss Empire, members of the Triple Tsad and allies of the Mandalorians.
The Mand'alor Empire used the Silver Castle to destroy Telos. Using the Silver Castle was just telling the galaxy of the firepower and destruction the Mandalorians could bring out. It was the first time an entire planet was destroyed in the galaxy.