The battle of Antietam was fought on September 17, 1862. The Army of the Potomac led by McClellan fought against the Army of Northern Virginia led by Lee. The battle was fought along the Antietam Creek in Sharpsburg, Maryland. It was a very bloody battle. The Union casualties were around 12,000, the Confederate casualties at around 10,000. In the end, Lee was forced to retreat. His intent was to cause a blow to the Union's morale with a victory on Northern soil, but, unfortunately for the Confederacy, he failed. Securing a location in Maryland would also give him a fair chance at attacking major Union cities nearby.
Had the Confederate Army of Northern Virginia been successful in its attempt, the European nations may have become involved, since many of them had a keen interest in the Confederacy from a financial standpoint. Great Britain or France might have tried to mediate in the dispute.
The Confederates were probably hoping for military and/or financial support (as they had a quite limited number of soldiers and funds), though it would seem that Great Britain would not be anxious to come back to North America and fight another war.
At Antietam, Lee suffered his first defeat. It was a great one as the casualties suggest. Had the South been able to retreat with minimal losses, they may have been able to reform into a more offensive position and continue on towards Washington. McClellan, being, as always, quite slow to act, could possibly have won the entire war on this day. By choosing to hold back three quarters of his men he was unable to deal a decisive defeat to Lee's army. Had McClellan attack with a full force, he might have been able to drive Lee's army back to Richmond and quickly stop the...