A classic of AH, right up there with the Second World War. The most popular ideas pivot around the Confederate States of America winning the American Civil War - as VoCSe once put it, “The most original What If ever”. Done to death as a topic of AH, threads nonetheless still crop up about it, and occasionally a new idea is produced.
Numerous ACW scenarios involve foreign intervention by European powers, usually on the side of the CSA. The most common of these is the Trent Affair - the taking of Confederate diplomats from the neutral British ship RMS Trent by Union Navy forces - instead of being smoothed over as one of the last acts of Prince Albert, turning into a war between the USA and the British Empire, sometimes also including Napoleon III's France as an ally.