So, assume that Abraham Lincoln does not get elected in 1860, and Stephen Douglas becomes president instead. He throws some bones to WASP conservatives, maybe even negotiates a provision that would allow for gradual abolishment of slavery in the southern states with compensation for the planters. South Carolina's secession doesn't happen, Fort Sumter doesn't happen, millions of lives are not lost, infrastructure is not destroyed, etc. Overall, the US, while perhaps less racially or socially equal than IOTL, maintains its strength for the remainder of the second half of the 19th century.
What would be the effects on the world of an USA that is not distracted with a large scale civil war and is, thus, more able to enforce the Monroe Doctrine and its other foreign policies?
One pivotal region, IMO, would be Mexico. The
McLane-Ocampo Treaty would have infused Benito Juarez' rebel liberal government and army with much needed cash for fighting in the Reform War, but it wasn't ratified IOTL because of the outbreak of the ACW. Here, it is -- and the French intervention in Mexico might even be avoided, leaving the republic in a much better financial position. What happens next for Juarez and Mexico?
Without a "Mexican Ulcer", how is Napoleon III's French Empire affected? Would he choose to intervene in the Second Schleswig War, if it still happens, to protect Denmark's interests?
What about Alaska? Could it be sold earlier than IOTL?
Any other thoughts? How does this affect the ongoing colonization of Africa?