Final answer:
The Emancipation Proclamation deterred foreign support for the Confederacy by reframing the American Civil War as a fight against slavery, which clashed with anti-slavery sentiments in Europe, notably in Britain.
Step-by-step explanation:
The Emancipation Proclamation issued by President Abraham Lincoln in 1863 had significant effects on the American Civil War and on international diplomacy, as it shifted the war's focus to the abolition of slavery. By declaring freedom for all slaves in the Confederate-held territories, the Proclamation made foreign intervention on behalf of the Confederacy highly unlikely, particularly from Britain and France, both of which had strong anti-slavery sentiments.
The British public and government, having abolished slavery in 1833, could not reconcile the support of the Confederacy with their stance against slavery. This strategic move by Lincoln not only bolstered the moral cause of the Union but also dissuaded any potential foreign allies the South had hoped to gain. As the Proclamation made its intent clear, the inclusion of nearly 200,000 African American soldiers into the Union army was encouraged, adding significant manpower and contributing to the Union's victory.