After annexing Bessarabia and Northern Bukovina from Romania in the summer of 1940, the Soviet Union gained a border on the Danube.

[4] The first Soviet-Romanian naval engagement occurred on 22 June 1941, the first day of Operation Barbarossa, when one Soviet monitor and one patrol boat attacked the port-city of Tulcea.

The Romanian river monitors Basarabia and the Brătianu-class Mihail Kogălniceanu repulsed the attack, sinking the patrol boat.

[2][5] The main naval engagement on the Danube occurred on the following day, when the Romanian Tulcea Tactical Group (Basarabia, Mihail Kogălniceanu and four patrol boats) repelled another attack of the Soviet Danube Flotilla, damaging two monitors and two patrol boats and one tug.

[7] Artillery duels with the Romanian Flotilla also continued until 9/10 July when the Soviet ships retreated to Izmail.