The Danube River forms the border of some countries. They are Romania, Bulgaria, Hungary, Serbia, Slovakia, Austria and Germany.
Slovakia and Hungary
Its the Danube river and Danube plain separates Romania and Bulgaria. There are no mountaneous border between the mentioned two countries. And Carpathian mountain range of Romnia or Balkan mountain range (most northern mountains of Bulgaria: none of them serves as border.
river Danube (река Дунав)
The Danube.
A border separating two countries is called an international border or a national border.
This river is called Danube. Its a northern border to Bulgaria and for Romania the south bound.
The two countries that border Andorra are Spain and France.
The two countries that border Somalia are Ethiopia and Kenya.
Do you mean countries? If you do, the two countries that border the U.S. are Canada and Mexico.
Canada and Mexico are the two countries that border the United States.
The two rivers that form the northern border of the Roman Empire are the Rhine and the Danube. The Rhine flows primarily through modern-day Germany and the Netherlands, while the Danube traverses several countries in Central and Eastern Europe, including Germany, Austria, and Romania. Both rivers served as significant natural boundaries for the empire, facilitating trade and military defense.