After The Nazi Army juggernauted it's way through Poland, Hitler cast his eyes to France for the taking. Seen as retribution for France's participating in The Treaty of Versailles (which saw Germany's defeat in World War One), Hitler occupied France in 1940. Not all French so easily surrendered. Large pockets of Resistance Fighters arose. Using effective Guerilla tactics, they kept The German Army in a relative state of constant alert. Some members of The French Army (in exile), established their resources in England as well. Many of their Air Forces helped defend England during The Battle of Britain. The Free French Flag was actually the Flag of Lorraine (in Eastern France). Consisting of the National Tricolor of three vertical lines of Blue, White and Red - The Center White Stripe featured a Red two-bar Cross, known as "The Cross of Lorraine". This symbol goes back to 12th Century France, and Lorraine's participation in battle against Charles The Bold. It is speculated that it was borrowed from Byzantine-Orthodox Catholic sources. The Flag was used on Free French Fighter aircraft, Armors and to identify it's "uniformed" soldiers. Often, the simple Cross was used to identify members of the "Underground" Free French Fighters (within occupied France).