The British were politically neutralized after the Battle of Antietam though, the victory there such as it was destroyed the view that the CSA could gain a clear or significant military advantage in any way, Antietam played a lot bigger in the papers then it played out on the battle field, which says a lot for the bloodiest single day in American military history. The victory at Antietam gave the Union a very much needed moral boost, put the Army of Northern Virginia back on a defensive footing while it licked its wounds, THEN most importantly gave Lincoln the boost he needed to smack down the Democrats in congress and continue his wartime policies, cinched the reelection for him and finally gave him the political fuel needed to sign the Emancipation Proclamation, which was the last nail in the coffin of any hope the Confederacy had for forgein intervention. The French wouldn't act without Britain, and once the Proclamation was signed the British population was overwhelmingly in support of the yan... err Union. All that was left then was to win the war, but Antietam was the critical turning point. Gettysburg was a vitally important battle, there is no doubt or argument there, but the twenty two thousand men who died or were wounded in the fields of Maryland died at the absolute tilting point of the American Civil War. No other single campaign or victory had nearly the lasting and wide scale effect as Antietam across Clauswitz's three pillars of warfare.
Not to mention the preeminent scholar on the American Civil War agrees with me
No other campaign and battle in the war had such momentous, multiple consequences as Antietam. In July 1863 the dual Union triumphs at Gettysburg and Vicksburg struck another blow that blunted a renewed Confederate offensive in the East and cut off the western third of the Confederacy from the rest. In September 1864 Sherman's capture of Atlanta reversed another decline in Northern morale and set the stage for the final drive to Union victory. These also were pivotal moments. But they would never have happened if the triple Confederate offensives in Mississippi, Kentucky, and most of all Maryland had not been defeated in the fall of 1862.
— James M. McPherson, Crossroads of Freedom
Don't make me publish my full paper at you jwhouk, I will do it.