 Under the Romans the Danube formed the border of the Empire with the tribes to the north almost from its source to its mouth.At the same time it was a route for the transport of troops and the supply of settlements downstream.From AD 37 to the reign of the Emperor Valentinian I (364–375) the Danubian Limes was the northeastern border of the Empire, with occasional interruptions such as the fall of the Danubian Limes in 259.