When the document was published, Northerners were outraged by what they considered a Southern attempt to extend slavery. American free-soilers, recently angered by the strengthened Fugitive Slave Law (passed as part of the Compromise of 1850 and requiring officials of free states to cooperate in the return of slaves), decried as unconstitutional what Horace Greeley of the "New York Tribune" labeled "The Manifesto of the Brigands." During the period of Bleeding Kansas, as anti- and pro-slavery supporters fought for control of the state, the Ostend Manifesto served as a rallying cry for the opponents of the Slave Power. The incident was one of many factors that gave rise to the Republican Party, and the manifesto was criticized in the Party's first platform in 1856 as following a "highwayman's" philosophy of "might makes right." But, the movement to annex Cuba did not fully end until after the American Civil War.