The Battle of the Heligoland Bight was the first "named" air battle of the Second World War, which began the longest air campaign of the war, the Defence of the Reich. On 3 September 1939, the United Kingdom declared war on Nazi Germany after the German invasion of Poland, which started the European War. The British did not assist Poland by land or sea but RAF Bomber Command flew several missions against German targets. A number of these air raids were directed at Kriegsmarine (German Navy) warships in German ports to prevent their use in the Battle of the Atlantic. With the front lines static between September 1939 and May 1940, a period known as the "Phoney War" set in, with little fighting on land or in the air.