On 3 September 1939, the United Kingdom formally declared war on Germany, marking the nation's entry into the Second World War. This momentous decision followed the German invasion of Poland, which had commenced two days earlier on 1 September. In response to this act of aggression, the British government issued a firm ultimatum to the German leadership, demanding the immediate withdrawal of their forces from Polish territory. When the deadline for this ultimatum passed without a satisfactory response, the state of war was confirmed.

The declaration of war was a pivotal moment that fundamentally altered the course of the twentieth century, drawing the United Kingdom and its vast empire into a global conflict. The move was broadcast to the nation by Prime Minister Neville Chamberlain, who informed the British public that all efforts to secure peace had failed. This announcement signalled the end of the period of appeasement and the beginning of a total mobilisation of British society, industry, and military resources to confront the threat posed by the Third Reich.

The events leading up to this declaration were characterised by a rapid escalation of tensions across Europe following years of diplomatic strain. The failure of the German government to respect the sovereignty of its neighbour left Britain with little choice but to honour its security guarantees to Poland. The subsequent entry into the war initiated a period of profound change for the British people, setting the stage for the challenges and hardships that would define the following six years of conflict.

• Germany invaded Poland on 1 September 1939.
• The British government issued an ultimatum demanding a German withdrawal.
• The deadline for the ultimatum expired on 3 September 1939.
• Britain declared war, officially entering the Second World War.
• The declaration committed the United Kingdom and its empire to the conflict.