- "The decades-long war between the Galactic Republic and the rejuvenated Sith Empire ended without a decisive battle. Instead, the conflict halted with a peace agreement neither side believed would last–the Treaty of Coruscant, a document forbidding Republic military activity within Imperial space and vice versa. Rules of engagement in neutral territory are far more vague, allowing for a range of border conflicts and attacks via third parties, but the treaty has prevented all-out warfare on more than one occasion. Negotiations began when the Empire–whose early victories had led to dwindling resources and overexpansion–reached out to a battered Republic and called for a summit on Alderaan. As the summit began, Imperial forces launched a surprise attack on Coruscant, the Republic capital. Conquering the city-world was impossible, but the Empire wreaked enormous havoc in a short period, effectively holding Coruscant hostage and forcing the Republic to agree to less-than-favorable terms."
- ―In-game Codex (Codex/Lore)
The Treaty of Coruscant marked the end of the Great Galactic War between the Galactic Republic and the Sith Empire. The two factions met on Alderaan to discuss the terms of peace, while the treacherous Sith launched a simultaneous attack against the Republic on their home world of Coruscant. The sacking and occupation of Coruscant allowed the Empire to force the Republic into a completely helpless position, leaving them no choice but to surrender to the Empire's terms. The Republic gave up control of multiple star systems and trade routes to the Sith Empire and gained little to nothing in return. This treaty marked the beginning of a Cold War.
All dates were adjusted to a new time system after the treaty was signed. So every event that takes place after the treaty would be ATC (After Treaty of Coruscant) while the events that have happened before would be BTC (Before Treaty of Coruscant). For example, the game itself takes place at 10 ATC, as stated in the Blood of the Empire webcomic on the main site.
You get this Lore during the Flashpoint Black Talon after Satele Shan speaks to your group.