The Phoney War (Phony War in United States), or in Winston Churchill's words the "Twilight War", was a phase in early World War II marked by few military operations in Continental Europe, in the months following the German invasion of Poland. Although the great powers of Europe had declared war on one another, neither side had yet committed to launching a significant attack, thus there was relatively little fighting on the ground. The term has equivalents in many other languages, notably the German Sitzkrieg ("sitting war," a pun on Blitzkrieg), the French drôle de guerre ("funny war" or "strange war," drôle having two meanings) and the Polish dziwna wojna ("strange war"). In Britain the period was even referred to as the "Bore War" (a pun on "Boer War").

While most of the German army was fighting against Poland, a much smaller German force manned the fortified defensive lines along the French border (Westwall). At the Maginot Line on the other side of the border, British and French troops stood facing them, but there were only some local skirmishes. The British Royal Air Force dropped propaganda leaflets on Germany and the first Canadian troops stepped ashore in Britain, while western Europe was in a strange calm for seven months. Meanwhile, the opposing nations clashed in the Norwegian Campaign. In their hurry to re-arm, Britain and France had both begun buying weapons from manufacturers in the USA at the outbreak of hostilities, supplementing their own productions. The United States, technically neutral in the war effort, contributed to the Western Allies by discounted sales, and, later, lend-lease, of military equipment and supplies. It should be noted that in the 1930s, private companies in Britain and the US were also supplying Germany, without government sanction. Engines of a few German fighters were made in Britain and raw materials were being sold in America to Germany. German efforts to interdict the Allies' trans-Atlantic trade at sea ignited the Second Battle of the Atlantic.