The Empire's attack on Belsavis came as a complete surprise. So far as the Republic knew, the prison's location was an absolute secret‒and even if the secret were exposed, the planet seemed unlikely to become a top-priority military target. In this, the strategists were mistaken.
The Empire began its attack by delivering a small team of Sith and elite soldiers onto the planet surface. Their first task was to free as many prisoners as possible and throw the prison into chaos. Some of the prisoners (including long-lost Imperial prisoners-of-war) allied with the Empire, while others joined the riots and attacked all parties. As the Republic scrambled to respond, the Empire established a makeshift base of operations on the surface and brought a mobile battle platform into orbit.
The Empire's position on Belsavis is precarious, but until the Republic can alter its focus from the prisoners to the invaders, the situation is unlikely to change.