The Second World War is conventionally considered to have begun in Europe on 1 September 1939, when Nazi Germany invaded Poland. Germany's invasion followed a period of territorial expansion and diplomatic crisis in which Nazi Germany had already annexed Austria and occupied parts of Czechoslovakia. Britain and France had guaranteed Polish independence earlier in 1939, and after Germany invaded Poland they declared war on Germany on 3 September. The European conflict therefore became an international war within days.
However, the exact global starting point can be interpreted differently because Japan had already been engaged in major warfare in China since 1937. The conventional date of 1 September 1939 refers specifically to the beginning of the European war that developed into the global conflict.
Germany's invasion was enabled by the Molotov Ribbentrop Pact of August 1939 between Germany and the Soviet Union. The agreement included a secret protocol dividing parts of Eastern Europe into spheres of influence. Germany attacked Poland from the west on 1 September, while the Soviet Union invaded eastern Poland on 17 September. Poland was therefore attacked by two major powers and subsequently occupied and partitioned.
The German military employed combined arms tactics involving armour, motorised forces and air power, although the popular term Blitzkrieg was not a formal German doctrine in the simple sense often presented. Poland's defeat demonstrated the immediate consequences of German military expansion and triggered British and French declarations of war.
Britain and France did not immediately send large armies into Poland because geographical and military circumstances limited their ability to intervene directly. The British government declared war at 11 am on 3 September 1939, followed shortly by France. The conflict was initially characterised on the Western Front by limited major ground combat, sometimes called the Phoney War. Germany then launched major offensives in Western Europe in spring 1940, rapidly defeating France and occupying much of the region. Britain continued fighting after France fell. The sequence demonstrates that the declaration of war and the beginning of major fighting were not necessarily simultaneous in every theatre.
The European starting date should be distinguished from earlier conflicts in Asia. Japan had invaded Manchuria in 1931 and established the puppet state of Manchukuo. Full scale war between Japan and China began in July 1937 after the Marco Polo Bridge Incident. Japan subsequently occupied large areas of China and committed major atrocities. Historians differ over whether the Second World War should be dated globally from 1937 because the war in China eventually became integrated with the broader conflict. In many European and Western chronological frameworks, however, 1939 remains the standard beginning because the German invasion of Poland produced the formal declarations that transformed the European crisis into a general European war.
The immediate diplomatic background involved Germany's rejection of the post First World War settlement and its programme of territorial expansion. Hitler had remilitarised the Rhineland in 1936, annexed Austria in 1938 and obtained the Sudetenland through the Munich Agreement later that year. In March 1939 Germany occupied the remainder of the Czech lands, violating the spirit and structure of the Munich settlement. This convinced Britain and France that further German expansion posed a serious threat. Their guarantee to Poland was intended to deter German aggression, but negotiations with the Soviet Union over a possible anti German alliance failed. The German Soviet pact then removed the immediate danger of Soviet intervention against Germany and facilitated the attack on Poland.
The outbreak also illustrates the difference between a long term process and a formal starting date. Aggressive nationalism, revision of postwar borders, economic instability, militarisation, ideological conflict and weaknesses in collective security had developed during the interwar period. The League of Nations had struggled to deter aggression in several international crises. Germany, Italy and Japan had challenged the existing international order through military expansion. These developments did not make war inevitable, because governments still made choices and diplomatic alternatives existed. Nevertheless, by 1939 the European security system had become increasingly unstable, and Germany's decision to invade Poland transformed the crisis into open war between major European powers.
The war expanded geographically after September 1939. Germany conquered Denmark and Norway in April 1940 and then attacked Belgium, the Netherlands, Luxembourg and France in May. Italy entered the war against Britain and France on 10 June 1940. Germany invaded the Soviet Union on 22 June 1941, dramatically expanding the conflict in Eastern Europe. Japan attacked Pearl Harbor on 7 December 1941, after which the United States entered the war against Japan. Germany and Italy subsequently declared war on the United States. By the end of 1941, separate conflicts in Europe, China and the Pacific had become a genuinely global war involving interlocking alliances and theatres.
Thus, 1 September 1939 is the conventional answer to the question of when the Second World War began, because that was the date Germany invaded Poland. Britain and France's declarations of war on 3 September marked the formal expansion of the European conflict into a war between Germany and the major Western European powers. Nevertheless, a comprehensive historical explanation must acknowledge the earlier Japanese war in China from 1937 and the preceding expansion of Germany, Italy and Japan. The choice of 1939 is therefore a chronological convention tied to the European outbreak, while the broader origins and development of the world war extended across several years and regions.
