The imposter Palpatine was responsible for the Imperial Civil War. Rooting himself on Imperial-held Mon Calamari, the imposter, a Dark Jedi, took control of a large amount of Imperial resources and military units, declaring he was Palpatine. He grew a large amount of support as many believed who he was. He then declared Bacharan Valak a traitor and went to war, aiming to seize control of the Empire. From SW1ki, a Wikia wiki.