In June of 1940, Nazis marched into Paris. France was well prepared for a frontal attack with their Maginot line, but did not expect Germany to come through the thick woods. France quickly fell and remained in German control through most of the war. The allied forces began liberating it starting with the D-Day Normandy landings, and by August 19th the Battle of Paris took place.
On this day August 25th, in 1944, the liberation of Paris is complete, with the surrender of the Nazis to allied forces. 3,200 were are killed, another 12,800 of the former occupiers are taken prisoner.
This marked the beginning of the end of Nazi control over Europe. Eventually the allied forces would go through all of Europe and liberate the numerous concentration camps. From the French point of view, it was the French themselves that liberated Paris as 20,000 Parisians stood up to the Nazi forces, resisting the occupation any way they could.