President Lincoln famously stated that he sent Federal troops to suppress the rebellion as a means to preserve the Union, and that freeing the slaves was not his aim at the outbreak of hostilities. Obviously, the elimination of slavery seems to have moved up his list of priorities by the time he signed the Emancipation Proclamation, but that was almost two years after the first shots were fired.

Those first two years included some of the bloodiest battles in American history. What was so urgent about preserving the Union that Lincoln was willing to tolerate those kind of casualties? I can find a lot of reasons for why the South wanted to leave, but it's hard to get a simple answer about why the North didn't simply let them.