During WW2 the Croats joined the Nazis thinking that they would join the winning side and gain land and independence and created an Independent Fascist Croat Regime (Ustase)which lead to the backstabbing of their slav brothers. This lead to the ethnic cleansing of Serbs much like that of the Jews in Concentration camps such as the notorious Jasenovac camp.
After the loss of the axis powers Croatia was still under Yugoslavia with Tito keeping everything under control. Tensions from the past world wars were still fresh between Croats and Serbs and after the death of Tito they declared independence. After a long war in 1991 they displaced as many non Croats as they could from their land and became their own country recognized by the world.
There are around 9 million Croats in the world with many in Bosnia (much of their own land) and Germany. More than half of their population doesn't live in Croatia due to their long history of conflict.