The Danzig crisis was a 1939 crisis that led to World War II breaking out in Europe.
Background
On 8 January 1918, the U.S. President Woodrow Wilson proclaimed the 14 Points as the American war aims. Point 13 called for Polish independence to be restored after the war and for Poland to have "free and secure access to the sea", a statement that implied the German deep-water port of Danzig (modern Gdańsk, Poland) located at a strategical location where a branch of the river Vistula flowed into the Baltic Sea should become part of Poland. Danzig was known as the "Amsterdam of the East" owing to its strongly Dutch-style architecture and its traditional role as a trading center that linked the markets of eastern Europe to the wider world. At the Paris peace conference in 1919, two of the "Big Three" leaders—Wilson and the French premier Georges Clémenceau—supported the Polish claim to Danzig, but the British Prime Minister David Lloyd George was opposed under the grounds that the population of Danzig was about 90% German.
In a compromise, it was agreed that Danzig would become a Free City that would belong to neither Germany nor Poland, but the latter was to have special rights in the city. The city-state would govern itself via a volkstag (people's assembly) while executive council was the Senate, but the League of Nations would appoint a Commissioner to oversee the affairs of the Free City. The Senate president served as the head of government for the Free City. The Polish delegation to the Paris peace conference led by Roman Dmowski had asked for the cessation of Danzig to Poland, and within Poland the creation of the Free City was widely seen as a betrayal of Point 13. The Polish position was always that Poland needed Danzig to be economically independent of Germany as Danzig was where most of Poland's exports and imports to the wider world went through. The loss of Danzig did indeed deeply hurt German national pride and in the interwar period, German nationalists spoke of the "open wound in the east" that was the Free City of Danzig. The precise legal status of Danzig was ambiguous as the American historian Elizabeth Clark noted: "...few experts, whether Polish, French or German, agreed on a legal description of the city, whetever it was a sovereign state, a state without sovereignty, a Polish protectorate or a League of Nations protectorate". The Free City had some of the markers of sovereignty such as its own police force, anthem, flag, currency and stamps.
In 1921, France and Poland signed a defensive alliance committing both powers to go to war in the event of an attack by Germany. The alliance with Poland was the cornerstone of the cordon sanitaire, as the French system of alliances in Eastern Europe were known, as France signed defensive alliances with Czechoslovakia in 1924, Romania in 1926 and Yugoslavia in 1927. At the time the alliances were signed, the Rhineland was demilitarized and occupied by the French Army, which was in a strong position to launch an offensive deep into Germany. The Rhineland, with the broad Rhine river and its steep hills, formed a natural defensive barrier and beyond the Rhineland was the wide open North German plain, which favored offensive operations. In June 1930, the French ended the occupation of the Rhineland five years earlier than the Treaty of Versailles had called for. As nobody in the French government actually expected the Germans to abide by the Treaty of Versailles, it was assumed that the Rhineland would be remilitarized at some point in the near future. The construction of the Maginot Line, beginning in 1929, was a tacit admission of this. The Maginot Line implied a defensive strategy in the event of war, which created a major strategical problem, namely how France would aid its allies in Eastern Europe should Germany turn east instead of west.