When Germany invaded Poland in 1939 Britain and France declared war on the former… but they didn’t actually do anything aggressive at first, breaking a promise to attack Germany within fifteen days and instead building up their armies for the long war they believed would happen. Some dubbed this inactivity the Phoney War, or punned on the German Blitzkrieg myth with Sitzkrieg. It would come to an end in May 1940 with the German conquest of France.