While both previous answers are valid to an extent, a close reading of Lincoln's writings as well as the historical context of the Emancipation Proclamation indicates the primary, driving force behind the War was the preservation of the Union. Lincoln, quite rightly, saw the oncoming disintegration of the relatively young ("four score and seven years ago...") Republic if it were allowed to fractionalize into regional mini-countries without a strong central, Federal government. For example, the Emancipation Proclamation, only applied to those southern States which had seceded, not to the slave holding border states. Additionally, it was not issued until September 22, 1862 and didn't become effective until January 1, 1863, nearly three years after the first secession... It was only later that slavery became the primary issue and some argue that it was never the real driving force behind the conflict...