The region that is now Croatia was originally occupied by Illyrians.  In the year 9 AD that region became part of the Roman Empire.  The Emperor Diocletian built a huge palace in Split (then Aspalata) where he retired from politics in 305 AD.  During the 5th century the last Roman Emperor Julius Nepos actually ruled from Diocletian's palace.  Migrating Slav clans arrived in what is now Croatia around the early 7th century.

The Kingdom of Croatia continued to exist from its first recognition by Byzantium in 925 AD until the end of World War I. Although it started as an independent kingdom it was later, partly as a defensive measure against the advance of the Ottoman Empire, included in multiethnic empires such as Hungary, the Habsburg empire and the Austro-Hungarian Empire.

The lands that today comprise Croatia were part of the Austro-Hungarian Empire until the close of World War I.  In 1918, after the end of World War I, the Croats, Serbs, and Slovenes formed a kingdom known after 1929 as Yugoslavia. During World War II the Nazi forces of Germany and Italy established the Ustaša regime in Croatia, which introduced anti-Semitic laws and massacred Serbs and other non-Croats who had been resident in the region.  The Partizani (partisans) established a strong resistance movement against Nazi forces, initially as guerrillas and then as a major army 800 thousand strong.

Following World War II, the re-established Yugoslavia became a federal independent communist state controlled by Marshal Josip Broz Tito, whose father was Croat and mother Slovene.   Croatia declared its independence from Yugoslavia in 1991, but it was not until 1998 that occupying Serb forces finally left their last enclave in Croatia, along with most of the ethnic Serb population in Croatia.

Croatia applied for EU membership in 2003 and was in negotiations from 2005 until 2011. Croatia joined NATO in April 2009; ratified the EU Accession Treaty in January 2012. It became the 28th EU member state on 1 July 2013, after all other 27 EU members had ratified the accession treaty.