Why did Poland's border change so much after World War 2? (Short Animated Documentary)

Why did Poland's border change so much after World War 2? (Short Animated Documentary)
Poland's borders changed quite a lot after World War 2, moving hundreds of miles to the west and massively altering the shape of the country. But why did this happen and why did Poland lose and receive the lands that it did? To find out watch this short and simple animated history documentary.