>In early 1941, Klingenberg was with SS troops taking part in the invasion of Yugoslavia. The objective was to push into Yugoslavia quickly, to then attack Greece. Klingenberg disobeyed orders, and decided to reconnoiter Belgrade, with his units far ahead of the main German army. Finding a boat, he made across the river, with the intention of ferrying a sizable force across; however, the boat sank, and he was left with just six men. Klingenberg then encountered some Yugoslav troops, who had captured a drunk German tourist, whom they in turn captured.
>After a number of firefights the six Germans, having sustained no casualties but capturing a number of Yugoslav soldiers, made it to the centre of Belgrade with the tourist and their prisoners. There they raised a German flag. The Mayor came out to meet them, after Klingenberg bluffed, telling him there was an incoming artillery barrage and an impending Luftwaffe attack. The Mayor, and some garrison troops surrendered the city to them on 13 April.
>At this point a few more of Klingenberg's men arrived the same way he had, and made a show of their presence, pretending that there was more of them than there were.[3] [4][5] The German army eventually arrived, dumbfounded at the situation, having made a complex plan to take the city that was no longer needed, and was expected to cost thousands of lives.[1][2][6] A few days later Yugoslavia surrendered.[1][2] Klingenberg was awarded the Knight’s Cross for capturing the city, in effect capturing Belgrade with just 6 men.