A declaration of war is a formal act by which one nation goes to war against another. The declaration is a performative speech act (or the signing of a document) by an authorized party of a national government in order to create a state of war between two or more states.

The legality of who is competent to declare war varies between nations and forms of government. In many nations, power is given to the head of state or sovereign; in other cases, something short of a full declaration of war, such as a letter of marque or a covert operation, may authorise war-like acts by privateers or mercenaries. The official international protocol for declaring war was defined in the Hague Convention (III) of 1907 on the Opening of Hostilities.

Since 1945, developments in international law such as the United Nations Charter, which prohibits both the threat and use of force in international conflicts, have made declarations of war largely obsolete in international relations. In addition to this, non-state or terrorist organisations may claim to or be described as "declaring war" when engaging in violent acts. These declarations may have no legal standing in themselves, but may still act as a call to arms for supporters of these organisations.

Arthur Neville Chamberlain FRS (18 March 1869 – 9 November 1940) was a British Conservative politician who served as Prime Minister of the United Kingdom from May 1937 to May 1940. Chamberlain is best known for his appeasement foreign policy, and in particular for his signing of the Munich Agreement in 1938, conceding the Sudetenland region of Czechoslovakia to Germany. When Adolf Hitler continued his aggression by invading Poland, Britain declared war on Germany on 3 September 1939, and Chamberlain led Britain through the first eight months of the Second World War.

After working in business and local government and after a short spell as Director of National Service in 1916 and 1917, Chamberlain followed his father and older half-brother in becoming a Member of Parliament in the 1918 general election at age 49. He declined a junior ministerial position, remaining a backbencher until 1922. He was rapidly promoted in 1923 to Minister of Health and then Chancellor of the Exchequer. After a short Labour-led government, he returned as Minister of Health, introducing a range of reform measures from 1924 to 1929. He was appointed Chancellor of the Exchequer in the National Government in 1931.

Adolf Hitler (German: [ˈadɔlf ˈhɪtlɐ] ( listen); 20 April 1889 – 30 April 1945) was an Austrian-born German politician and the leader of the National Socialist German Workers Party (German: Nationalsozialistische Deutsche Arbeiterpartei (NSDAP), commonly referred to as the Nazi Party). He was chancellor of Germany from 1933 to 1945 and dictator of Nazi Germany (as Führer und Reichskanzler) from 1934 to 1945. Hitler is commonly associated with the rise of fascism in Europe, World War II, and the Holocaust.

A decorated veteran of World War I, Hitler joined the German Workers' Party, precursor of the Nazi Party, in 1919, and became leader of the NSDAP in 1921. In 1923, he attempted a coup d'état, known as the Beer Hall Putsch, in Munich. The failed coup resulted in Hitler's imprisonment, during which time he wrote his memoir, Mein Kampf (My Struggle). After his release in 1924, Hitler gained popular support by attacking the Treaty of Versailles and promoting Pan-Germanism, antisemitism, and anticommunism with charismatic oratory and Nazi propaganda. After his appointment as chancellor in 1933, he transformed the Weimar Republic into the Third Reich, a single-party dictatorship based on the totalitarian and autocratic ideology of Nazism. His aim was to establish a New Order of absolute Nazi German hegemony in continental Europe.