The border disagreement along the Danube River valley started in 1947 but was left unresolved during the existence of the Socialist Federal Republic of Yugoslavia. The disagreement got bigger after Yugoslavia broke up. Serbia thinks the thalweg of the Danube valley and the center line of the river is the international border between the two countries. The Croatia government disagrees and claims that the international border lies along the old lines of the cadastral municipalities along the river — as the Danube was in the 19th century before meandering and hydraulic engineering works altered its course. As a result, Croatia claims a large part of the area controlled by Serbia, while Serbia does not claim the smaller parts on the other side.