The monument is the Maine Monument situated in Central Park NYC.
It is a memorial to the sailors who died when the battleship USS Maine blew up while in Havana harbour Cuba in January 1898. The Maine was there on a goodwill visit after earlier tensions between the USA and the Spanish possession of Cuba. A massive explosion ripped through the ship killing 260 men of the 355 man crew.
The USA started a Court of Inquiry and rather rapidly concluded that the cause of the explosion was due to a mine placed beneath the ship and on April 21st the USA declared war on Spain.