Short answers, pulled from the story.
France declared war on Germany on the 3rd of September 1939, when its ultimatum to Berlin expired at 17:00. Germany had invaded Poland two days earlier, on the 1st of September, and the United Kingdom had made its own declaration of war earlier that same day.
France declared war on Germany in fulfillment of a defensive treaty with Poland after Germany invaded the country on the 1st of September 1939. The French declaration stated that France and England had made countless efforts to safeguard peace, including a final appeal to Germany on the morning of September 3rd, which Germany refused.
The French declaration described Poland as "the victim of the most brutal and most cynical of aggressions." It stated that Polish cities were being bombed and that the Polish army was heroically resisting the invader.
France placed the entire responsibility for the bloodshed on the Hitler Government. The declaration stated that the fate of peace had been in Hitler's hands and that he chose war.
The French declaration stated that Germany desired the destruction of Poland in order to dominate Europe quickly and ultimately to enslave France. It also accused Germany of refusing to respond to all those who had raised their voices for peace.
The declaration described France's cause as "identical with that of Righteousness" and "the cause of all peaceful and free nations," predicting that it "will be victorious." France cast itself as a nation compelled to act, stating that it was waging war "because it has been thrust on us."