[lolinder]

The reason why both countries refuse to claim ownership is actually really interesting: both sides agree that the Danube should be considered the border, but the Danube has changed course over the centuries in a way that left a lot more land on the east than was there before. Naturally, that means that Croatia insists on using the historical path of the Danube, while Serbia insists on the modern one. For either side to claim the land on the west of the current Danube would be to cede the larger quantity of land to the east.