 After the establishment of the Kingdom of Serbs , Croats and Slovenes , the HSS established itself as the most popular Croatian political party and was very popular despite efforts to ban it . The 1921 constitution defined the kingdom as a unitary state and abolished the historical administrative divisions , which effectively ended Croatian autonomy ; the constitution was opposed by HSS . The political situation deteriorated further as Stjepan <unk> of the HSS was assassinated in the Yugoslav Parliament in 1928 , leading to the dictatorship of King Alexander in January 1929 . The HSS , now led by <unk> <unk> , continued to advocate the <unk> of Yugoslavia , resulting in the <unk> – <unk> Agreement of August 1939 and the autonomous <unk> of Croatia . The Yugoslav government retained control of defence , internal security , foreign affairs , trade , and transport while other matters were left to the Croatian Sabor and a crown @-@ appointed Ban . This arrangement was soon made obsolete with the beginning of World War II , when the Independent State of Croatia , which banned all political opposition , was established . Since then , the HSS continues to operate abroad . 
