Great Britain was deeply affected by the American civil war since American cotton was such an important part of the burgeoning British textile manufacturing industry, and the supply of this was hugely disrupted by the Union blockade of the South. The South, in turn, was eager to access British manufactured arms in order to defend itself from the North as it lacked the native industrial capacity to compete with the more heavily industrialized North on its own.

Britain recognized the belligerency of the South, but didn't recognize it as a nation, sign any treaties, or exchange diplomats. The Prime Minister at the time set Britain's official posture as neutral due to more Euro-centric strategic concerns (Bismarck and the beginning of a rise of a united Germany), but it would not be surprising if economic or military leaders favored intervening on the side of the Confederacy for a number of reasons. British popular opinion favored the Federal Government, however.

One thing is for sure though, Lincoln was deeply troubled by the prospect of British intervention, and that factored into the political calculation of the Emancipation Proclamation. The British Empire had outlawed the practice of slavery in 1833, so political or military support of a slave nation was a liability. Following the Union "victory" at Antietam (Lincoln calculated that he needed a victory before proclaiming emancipation else it would appear desperation) he promulgated the decree freeing all slaves held in bondage in the areas still under rebellion. It managed to free not a single slave upon proclamation, but succeeded in changing the narrative of the war away from a rebellion over constitutional disagreements toward a contest over the legitimacy of slavery as an institution, and undercut any chances that Great Britain would ever intervene on the side of a slave holding nation. It should rightly be seen as a masterful blend of foreign and domestic policy that was the central pivot point of the American Civil war. All that remained was a military victory, for the political and diplomatic one had been won.