If the war was not about slavery, I wonder why the south didn’t make a treaty with England to sell cotton to them exclusively and free the slaves in exchange for military assistance.
Very good point. Actually, the South had attempted all along to obtain recognition as a country by England, France and Spain. England was leaning that way (and France and Spain would likely have followed) until the South's losses at Gettysburg and Antietam. They then saw the writing on the wall and decided to support the north.
However, after that time, Judah Benjamin, Confederate Secretary of State, made several secret diplomatic trips to Europe to try to convince England to change its position, even offering emancipation of the slaves in return -- but by then it was too late.