World War II: Bulgaria and the Soviet Union successfully complete their defense of the north bank of the Drava River as the Battle of the Transdanubian Hills concludes.

The Tsardom of Bulgaria, also known as the Third Bulgarian Tsardom, usually known in English as the Kingdom of Bulgaria, or simply Bulgaria, was a constitutional monarchy in Southeastern Europe, which was established on 5 October [O.S. 22 September] 1908, when the Bulgarian state was raised from a principality to a tsardom.

The Union of Soviet Socialist Republics (USSR), commonly known as the Soviet Union, was a transcontinental country that spanned much of Eurasia from 1922 until it dissolved in 1991. During its existence, it was the largest country by area, extending across eleven time zones and sharing borders with twelve countries, and the third-most populous country. An overall successor to the Russian Empire, it was nominally organized as a federal union of national republics, the largest and most populous of which was the Russian SFSR. In practice, its government and economy were highly centralized. As a one-party state governed by the Communist Party of the Soviet Union (CPSU), it was the flagship communist state. Its capital and largest city was Moscow.

The Drava or Drave, historically known as the Dravis or Dravus, is a river in southern Central Europe. With a length of 710km, or 724km, if the length of its Sextner Bach source is added, it is the fifth or sixth longest tributary of the Danube, after the Tisza, Sava, Prut, Mureș and likely Siret. The Drava drains an area of about 40,154 square kilometers. Its mean annual discharge is seasonally 500m³/s to 670m³/s. Its source is near the market town of Innichen, in the Puster Valley of South Tyrol, Italy. The river flows eastwards through East Tyrol and Carinthia in Austria into the Styria region of Slovenia. It then turns southeast, passing through northern Croatia and, after merging with its main tributary the Mur, forms most of the border between Croatia and Hungary, before it joins the Danube near Osijek, in Croatia.