King Cotton is a decision available to the Confederate States of America if they are at war with the USA. That means, that it is available during the civil war, but it can technically be used in hypothetical later war.
This did not happen in real life, but it was considered. The decision is possible if the CSA's cotton reserves are greater than or equal to 500. It gives every great power (other than the USA) a Cut down to Size Casus Belli on the USA for the next 24 months. This decision is likely to be desired if the player is the CSA, because if any of the great powers follow through on the CB, it can force the USA to look somewhere else or be significantly weakened. There is however the risk, that no great powers react, and then 500 cotton are wasted.