The Soviet officer is Semyon Krivoshein, he's Jewish and not celebrating for multiple reasons. The Nazis occupied Brest when they weren't supposed to, Krivoshein got there and started negotiating to try to get them to leave. The Nazis were making a propaganda film out of this (which is where the picture comes from) so they wanted the Soviet army to have a parade with them. After an argument Krivoshein agreed that just him and some of his staff would stand there and watch the Nazis parade out of town in exchange for the Nazis leaving Polish prisoners they took in Brest.
Brest was 50% Jewish at this point, in Operation Barbarossa the Soviets defended for 6 days, Jewish Soviet officers were summarily shot by the Nazis, almost the entire Jewish population of Brest died in the Holocaust.
Even if you like the bot you should be downvoting it because that puts it in a predictable spot: at the bottom, without getting in the way of real comments.