During the early part of the American Civil War, in 1861, the Confederacy sent two ambassadors towards England and France aboard the British steam ship the Trent. The British ship was intercepted by the Union vessel USS San Jacinto. The ship was boarded, and the ambassadors, James Mason and John Slidell, were arrested and taken off the ship and jailed at Fort Warren.
England was infuriated by this violation of her sovereignty and the Laws of the Sea. Lord Palmerston prepared a heated missive which would have brusquely demanded the ambassador's release, and 11,000 British troops and additional guns were dispatched to Canada.
Queen Victoria's consort, the German Prince Albert, rewrote the memo before it was sent, softening the langauge and providing Abraham Lincoln with an honorable way out. British ambassador Lord Lyons delayed the delivery of the modified note until tempers cooled, and U.S. secretary of state, W.H. Seward apologized and released the ambassadors.
Had he not done so, it might have led to war between England and the Union, or England's recognition of the Confederate States of America.
This interesting possibility is addressed in Harry Harrison's Alternate History novel Stars & Stripes Forever.