The use of the name Montenegro began in the 15th century when the Crnojevic dynasty began to rule the Serbian principality of Zeta; over subsequent centuries Montenegro was able to maintain its independence from the Ottoman Empire. From the 16th to 19th centuries, Montenegro became a theocracy ruled by a series of bishop princes; in 1852, it was transformed into a secular principality. After World War I, Montenegro was absorbed by the Kingdom of Serbs, Croats, and Slovenes, which became the Kingdom of Yugoslavia in 1929; at the conclusion of World War II, it became a constituent republic of the Socialist Federal Republic of Yugoslavia. When the latter dissolved in 1992, Montenegro federated with Serbia, first as the Federal Republic of Yugoslavia and, after 2003, in a looser union of Serbia and Montenegro. In May 2006, Montenegro invoked its right under the Constitutional Charter of Serbia and Montenegro to hold a referendum on independence from the state union. The vote for severing ties with Serbia exceeded 55% - the threshold set by the EU - allowing Montenegro to formally declare its independence on 3 June 2006.