The War of Secession, 1 ABY-19 ABY or Confederate War of Independence, was a conflict between the Galactic Empire and several former Imperial systems, the Galactic Confederate Systems, and concluded in a galactic war that almost beat the Clone War as the bloodiest war in galactic history.
The war was the culmination of the political Confederate Revolution, whereby many of the systems rejected the legitimacy of Emperor Palpatine's rule, who had committed many atrocious sentient rights violations since he declared himself Emperor. The First Confederation Congress met in 0 BBY to coordinate relations with the Emperor and the by-then thirteen self-governing and individual systems, petitioning Palpatine to cease his genocidal and discriminating policies, organizing a boycott of Imperial goods, while reinstating their loyalty to the Imperial Throne.
Their pleas were not only ignored, they were punished with mass executions. With Imperial soldiers slaughtering many civilians throughout the galaxy, by 0 ABY the Congresses formed the Second Confederation Congress and authorized a Confederation military. Additional petitions to the emperor for in the following year with Congress being declared traitors and the systems to be in rebellion. The Confederates responded in 1 ABY by formally declaring their independence and secession as one new nation — the Galactic Confederate Systems — claiming their own sovereignty and rejecting any allegiance to the genocidal and oppressive Imperial Sith monarchy.
The Rebel Alliance's government under Mon Mothma secretly provided supplies, ammunition and weapons to the rebels starting in 2 ABY, and the Confederate's capture of an Imperial army in 2 ABY led the Alliance to openly enter the war in early 3 ABY, which evened the military strength with the Empire. The Smugglers' Alliance, Jabba the Hutt's Empire and Rebel Alliance allies also went to war with the Imperial over the next two years, threatening an invasion of Coruscant and severely testing Imperial military strength with campaigns in the Inner Rim — including attacks on Minorca and Gibraltar — and an escalating global naval war. Smugglers' involvement culminated in the expulsion of armies from the Outer Rim, securing the Confederate systems' southern flank.
Rebel involvement in naval battles proved very victorious, with a Rebel naval victory in the Sullust leading to the siege of Bastion in 19 ABY to the surrender of a second Imperial army. The Treaty of Independence ended the war and recognized the sovereignty of the Confederate Systems and the New Republic.