So, back in the olden days, there were two big groups of people fighting in a big war called World War II. One group was called the Allies and the other group was called the Axis. One of the Allies was a country called the Soviet Union, which is now called Russia. The Axis had taken over a fancy city called Vienna in Austria and the Soviet Union did not like that very much.
So, the Soviet Union decided to try and take back Vienna from the Axis. They did this by attacking with lots of soldiers and tanks and planes. It was a big battle, but eventually the Soviet Union won and took back control of Vienna. This was a very important victory in the war because it helped the Allies get closer to winning the whole thing.
So, the Vienna Offensive was a big battle between the Soviet Union and the Axis powers to take control of Vienna, and the Soviet Union ended up winning.