In 3653 BBY, while Imperial forces were engaged by the Republic's forces in the outer rim, Imperial forces attacked Coruscant and took it hostage after beating the Republic's forces.
The Sith Council then proposed the "Treaty of Coruscant," the terms of which included that the Republic's forces and the members of the Jedi Order were required to withdraw from battle against the Sith Empire by a deadline.
The treaty was accepted by the Galactic Republic and the Imperial forces left Coruscant free. After that, the Sith Empire started preparing for the next attack to conquer Coruscant.
My question: The whole point of the Great Galactic War was to take control of the Galaxy by capturing the core. The Sith Empire was successful in that when they took Coruscant hostage. What was the point of proposing the "Treaty of Coruscant"?
The Republic's forces were weak at that time. That's why they lost Coruscant, failed to clear Coruscant of the Empire's forces, and accepted the treaty. There wasn't any risk while keeping Coruscant hostage. After a period of time, they could always increase forces there to gain control over nearby space. Additionally, after connecting with main Imperial forces, it'd be a win-win situation. They would never have needed to prepare to capture Coruscant in the future again.