13 August 1920
Battle of Warsaw
The shape of the eastern border was decided by the Polish victory in the war with the Bolsheviks whose key battle – the Battle of Warsaw – took place in August 1920. It not only ended in a Polish victory but it is said to have stopped the march of the Bolsheviks to the west of Europe. Consequently, it is included on the list of battles that changed the course of world history. The war ended on 18 March 1921 with the Treaty of Riga.