What are the top reasons France entered World War 2?
France "entered" WWII because France and Britain had an alliance with Poland that said if Poland should be attacked then both Britain and France would declare war on the aggressor, which just happened to be Germany.
Why France actually bothered declaring war is an interesting question because the German army swept through to Paris in about 40 minutes, and once they were there told the French either to surrender or their beautiful Paris would be reduced to a crater. So they surrendered immediately, and installed a puppet government known as Vichy under Marshall Petain that did what the Germans told it to do, which included rounding up French Jews for deportation to the death camps. Petain was at one time sentenced to death for treason, but his death sentence was commuted by de Gaulle after the war.
Many patriotic Frenchmen (and an enormous number of women) ran an efficient underground movement returning downed allied pilots back to England, and running sabotage cells against German installations. Many of them were subjected to months of the most hideous of Gestapo tortures, and many of them to death, without revealing crucial information. We owe them a great debt of gratitude for their bravery. If we were to ever find out their names, the French book of war heroes would undoubtedly be thicker than it is.
There is more about Vichy here: http://en.wikipedia.org/wiki/Vichy_France Phil