After the Battle of Endor, once the Union was formed, Admiral Solo lead a two-year long campaign of aggressive, rapid-paced assaults on the remains of the Imperial fleet, wiping out all pockets of resistance. Grand Admiral Thrawn's fleet--still in the process of being amassed--was the first to fall. Thrawn went into hiding, but Bel Iblis tracked him down, to recruit him as the commander of the Coalition navy in his revolt against the Union. Thrawn was all too eager to accept.
You wanna know one thing I hate about Thrawn? That damn blue skin doesn't look good with anything! Nothing but his white Grand Admiral's uniform looks good on him!