"Finally, had Lee pulled off his invasion of Maryland and returned to Virginia without battle, it would have demonstrated to France and England the Confederacy's viability (like Saratoga did during the Revolution). Foreign recognition, aid and assistance would have made Union victory less likely. By 1863, it was already off the table for the European powers. England wouldn't and Napoleon III would not act without England's concurrent consent."
I think Lincoln could have effectively neutralized the threat of foreign intervention in the war, even had the South kept winning, by simply releasing the Emancipation Proclamation no matter what was happening.
The last thing England and France wanted was the war to be about slavery, for them to be seen to be supporting the cause of slavery.
There are good indications that Lincoln had the Emancipation Proclamation in his desk in at least a draft form as early as fall 1861.
Lincoln issued the preliminary proclamation in September 1862, at a time when the South was still fairly well in control on the battlefield.
From that point going forward it is very unlikely that Britain and France would have intervened even had Gettysburg been a Southern victory.
After Lincoln signed the proclamation on January 1, 1863, Europe was out of the war no matter what happened.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.