Alfred Thayer Mahan is an American event.
It requires that 
USA is a great power and 
Confederate States of America does not exist. Then it can happen between 1890 and 1930. Usually that means, that the American Civil War is either won by USA or completely prevented, but it is possible if they have lost, and CSA has disappeared in some other fashion.
It has a mean time to happen of 5 months, which decreases by 25% after 1891 and again by 1892 by 1892.
It gives USA 2 prestige.