 Complete the following phrase:  With the German invasion of Yugoslavia on 6 April 1941, the Royal Yugoslav River Flotilla was prepositioned on the Danube and Tisa rivers tasked to close off internal waterways to enemy infiltration and provide support to Royal Yugoslav Army land units . During the opening days of the invasion, the River Flotilla successfully carried out offensive operations against Axis forces shelling the German airfield at Mohács, Hungary on the 6 April 1941 and again two days later. On 11 April 1941, units of the flotilla withdrew towards Novi Sad under repeated attacks by German dive-bombers. In the early morning of 12 April 1941, a squadron of German Junkers Ju 87 dive-bombers attacked the Yugoslav river monitors near Čelarevo hitting the monitor Drava. Although several bombs that were unable to penetrate the vessels 300mm thick deck armour, one bomb hit the monitors funnel killing the ships captain Commander Aleksandar Berić and killing 53 of the 67 man crew. During the attack, anti-aircraft gunners claimed 12 enemy aircraft shot down, not including the Axis aircraft destroyed at Mohács. The remaining three Yugoslav monitors were scuttled by their crews on 12 April 1941 as German and Hungarian forces occupied their bases and the Yugoslav river systems. Soon thereafter, the Yugoslav River Flotilla ceased to exist with the 