historians are fighting over this concept, so it is to be determined.
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Initially - doubts about the viability of the Confederacy. They wanted to back a winner. Ethical position over slavery - although the war was not originally about slavery, the Lancashire cotton workers said they would sacrifice their jobs, rather than support the Confederacy. Lincoln's Emancipation Proclamation made it impossible for free nations abroad to support the South without looking pro-slavery themselves. Especially good diplomatic work by Lincoln's envoy in London - C.F. Adams (of the Presidential family).
He would rather keep the union together. with the south threatening to secede, he had no choice but to negotiate. He would keep slavery legal if it meant keeping America whole. He understood the how costly a war would be; and was.
The United States
No doubt both parties claimed that they disapproved of war. But by the time Lincoln got elected, there weren't many pacifists around on either side. The South was so sure it could beat the North that it was willing to risk a war, rather than accept Lincoln's ban on new slave-states. (Remember, Lincoln had not insisted on abolition, only on no extension of the slave-empire.) And some of Lincoln's cabinet were willing to fight the Confederates and the British at the same time, following the US naval capture of Confederate envoys from a British ship.
* They believed in the right of each state to detemine whether it allowed slavery (States' Rights) * They believed in the economic necessity of slavery * They saw themselves as citizens of their state, rather than citizens of a country made up of many states.