France gave Germany an ultimatum to withdraw its troops from Poland. When they received no response from Hitler, France after England declared war on Germany. France decided to keep its obligations to Poland and with Britain as its ally, was forced to declare war on Nazi Germany on September 3, 1939. Hitler had invaded Poland on September 1, 1939.
France entered WW2 because they were worried that Germany was going to attack them again, as they did in WW1. Adolf Hitler was swallowing up countries all around Germany, and it was natural to think he would try and conquer France.
Because they wanted to stop Germany
Additional view: France entered WWII because France (and Britain) had previously agreed to support Poland in the event of an attack on that country. In point of fact France actually invaded Germany shortly afterwards, but then pulled out.