Liberland
The official Liberland website says the nation was created on no-man's land (terra nullius) that emerged because Croatia and Serbia have not agreed on common borders for over 25 years. This border dispute includes some areas to the east of the Danube which are claimed by both Serbia and Croatia. Croatia considers some areas on the west side of the river, including Liberland, to be part of Serbia, although Serbia no longer claims that land.