exactly, the US government fought the war to maintain the union. The 13th Amendment did not have the support it needed to pass when Lincoln used an executive order to free the slaves. He recognized that freeing the slaves would spell the end for the confederacy by enticing slaves to leave their "owners". provide more soldiers and general cause chaos in the Confederacy. In the absence of the slaves the Confederacy was doomed, The system of slavery had already ensure their demised but abolishing slavery just moved up the time line. The US government was right to maintain the union and prevent the death of a democratic form of government that was unique in the world and abolishing slavery was a tool in that endeavor.