It is believed Celts settled Slovenia around 400BCE. In the year 10BCE the Romans conquered the area, founding many of its current cities like Ljubljana and Celje. When the Roman Empire collapsed in the 500's CE, the area was soon invaded by Slavic Peoples from the Ukraine. In the 700's the Franks took control of the region and by the 800's, Slovenia became part of the Holy Roman Empire. It was at this time that many of the people converted to Christianity. By the 1300's, Slovenia came under the domination of the Hapsburg Empire. Though there were some rebellions in the 15th and 16th century, Slovenia remained under the control of the Hapsburgs and the Austrian Empire. For a short period of time, when Napoleon controlled most of Europe, the area gained some degree of independence, but when Napoleon was defeated, Slovenia reverted to Austrian control, where it remained until the end of World War I. It was then that the Slovenians became part of Kingdom of the Serbs, Croats, and Slovenes, later to be known as Yugoslavia. Adolph Hitler conquered and controlled Yugoslavia during World War II. After the war, Yugoslavia became communist under General Tito. When Tito died in 1980, Yugoslavia slowly began to come apart. Then in 1991, Slovenia was the first Yugoslav Republic to declare independence. Slovenia was invaded in small part by the Serbians, but the EU stepped in and brokered a peace treaty. During the 90's Slovenia changed its economy to a free market economy and by 2004, became a member of the European Union. In 2007, they adopted the Euro currency.