The Anaconda Plan is a decision that is made during the American Civil War by the 
USA against the 
Confederate States of America. If it is not used during the civil war, it can technically be used in a hypothetical later war.
The results of this decision increase the war exhaustion of the CSA by 0.05 pr. month. It is recommended to use it especially if you are beginning to lose the war, but it is useful in any situation.
It is possible to enact it, when USA makes a blockade on at least 80% of CSA's provinces. It is very likely that USA does so during the civil war.
Breaking the Anaconda!
As soon as USA stops blockading at least 80% of CSA's ports, the event Breaking the Anaconda! will happen, which removes the effect giving 0,05 war exhaustion. Once the blockade is removed, it is not possible to retake the decision, so USA should only take the decision, once they are sure that CSA can't possibly break the blockade.
The event will also fire if CSA is at peace.