From Wikipedia

The Civil War
Episode Eight

War Is All Hell

Entry into the war by Britain and France on behalf of the Confederacy would have greatly increased the South's chances of winning independence from the Union. The Union, under Lincoln and Secretary of State William H. Seward worked to block this, and threatened war if any country officially recognized the existence of the Confederate States of America (none ever did). In 1861, Southerners voluntarily embargoed cotton shipments, hoping to start an economic depression in Europe that would force Britain to enter the war in order to get cotton. Cotton diplomacy proved a failure as Europe had a surplus of cotton, while the 1860–62 crop failures in Europe made the North's grain exports of critical importance. It was said that "King Corn was more powerful than King Cotton", as US grain went from a quarter of the British import trade to almost half.

When Britain did face a cotton shortage, it was temporary, being replaced by increased cultivation in Egypt and India. Meanwhile, the war created employment for arms makers, iron workers, and British ships to transport weapons.

Charles Francis Adams proved particularly adept as minister to Britain for the U.S. and Britain was reluctant to boldly challenge the blockade. The Confederacy purchased several warships from commercial ship builders in Britain. The most famous, the CSS Alabama, did considerable damage and led to serious postwar disputes. However, public opinion against slavery created a political liability for European politicians, especially in Britain. War loomed in late 1861 between the U.S. and Britain over the Trent Affair, involving the U.S. Navy's boarding of a British mail steamer to seize two Confederate diplomats. However, London and Washington were able to smooth over the problem after Lincoln released the two.

In 1862, the British considered mediation—though even such an offer would have risked war with the U.S. Lord Palmerston reportedly read Uncle Tom’s Cabin three times when deciding on this. The Union victory in the Battle of Antietam caused them to delay this decision. The Emancipation Proclamation further reinforced the political liability of supporting the Confederacy. Despite sympathy for the Confederacy, France's own seizure of Mexico ultimately deterred them from war with the Union. Confederate offers late in the war to end slavery in return for diplomatic recognition were not seriously considered by London or Paris.