Rule Britannia

In 1863 confederate president Jeff Davis outlaws slavery in the confederacy. Despite talking about doing the same, Abraham Lincoln does no such thing in the Union and three months later the British Empire declares war on the Union on the grounds of stopping the save trade.
In 1865 the American civil war draws to a close with much of East America once again under British rule.

By 1960 the world is split between the British and German empires. Global tensions are at breaking point and it seems nothing can prevent the world's first truly "world war".