1 September 1939: Why Britain Entered the War Two Days Later

By GlobeBids History Desk | Published 1 September 2026

Germany invaded Poland on 1 September 1939, transforming a European crisis into open warfare and forcing Britain to decide whether it would honour its commitment to Polish independence. Britain did not enter the conflict that morning: its declaration of war followed on 3 September, after Germany failed to meet a British demand to withdraw its forces.

The three-day sequence at a glance

• 1 September: German forces invaded Poland; Britain and France demanded an end to the attack.
• 2 September: Diplomatic exchanges continued, but Britain had not yet declared war.
• 3 September: A British ultimatum expired at 11 a.m., and Britain declared war on Germany.
• Later on 3 September, France also declared war on Germany.

Encyclopaedia Britannica identifies 1 September as the beginning of Germany’s invasion of Poland. Imperial War Museums places that attack within the diplomatic sequence that brought Britain and France into the war two days later.

Why the Anglo-Polish guarantee mattered

Britain’s response grew from decisions made months before the invasion. After German expansion had overturned earlier assurances about territorial ambitions, the British government announced a guarantee to Poland in March 1939. It promised support if Polish independence faced a clear threat and Poland resisted.

That political assurance was strengthened by the Anglo-Polish Agreement of Mutual Assistance, signed on 25 August 1939. Its purpose was partly deterrence: British leaders hoped that an explicit commitment would convince Germany that an assault on Poland could not remain an isolated conflict.

The guarantee did not place British troops in a position to stop the invasion immediately. Nor did it create an entirely automatic declaration at the moment German forces crossed the frontier. Britain still had to determine that the commitment applied, coordinate with France and decide how and when to issue its final demand.

Once the invasion began, however, retreating from the guarantee would have damaged British credibility and left Germany free to redraw another European frontier by force. The attack therefore became a test of both Polish independence and Britain’s stated policy.

Diplomacy moved from protest to ultimatum

German land and air forces attacked Poland early on 1 September. The campaign combined rapid armoured advances, infantry operations and air power, placing Polish forces under pressure across several fronts.

Britain and France called for German forces to leave Poland. For roughly two days, diplomatic activity continued while British ministers faced growing pressure to turn their promise into action. The absence of an immediate declaration on 1 September did not mean the invasion had been accepted; the British government was attempting to secure withdrawal before taking the final step into war.

On the morning of 3 September, Britain delivered an ultimatum requiring Germany to give an assurance that it would suspend hostilities and withdraw. No satisfactory response arrived before the deadline.

At 11 a.m. on 3 September, the ultimatum expired and Britain regarded itself as being at war with Germany. Prime Minister Neville Chamberlain informed the public in a radio broadcast shortly afterwards. France declared war later the same day, acting on its own commitments to Poland.

This distinction between the two dates is essential: 1 September marks the German invasion and the conventional beginning of the Second World War in Europe, while 3 September marks Britain’s formal entry into that conflict.

The global war did not begin everywhere at once

Calling 1 September 1939 the start of the Second World War is useful, but it requires geographical context. Major fighting was already taking place in East Asia, including the war between Japan and China that had expanded in 1937.

Britain’s declaration also did not mean that every part of what became the Allied coalition entered on the same day. The British decision immediately affected the United Kingdom and much of its empire, while self-governing dominions followed their own constitutional processes. Canada, for example, declared war later in September.

The conflict remembered as a single world war was therefore formed through connected invasions, declarations and campaigns rather than one simultaneous global announcement. The events of 1–3 September were decisive because they turned Germany’s attack on Poland into a wider European war involving Britain and France.

Mobilisation and evacuation brought the crisis into British homes

For people in Britain, preparations had begun before Chamberlain’s 3 September broadcast. Reservists were being called up, civil-defence organisations were activated and households had already been supplied with gas masks. Air-raid precautions reflected the widespread expectation that German bombing might begin as soon as war was declared.

The organised evacuation commonly known as Operation Pied Piper began on 1 September. Schoolchildren and other vulnerable civilians were moved from London and additional towns considered likely targets. Many travelled carrying small bags, identity labels and gas-mask boxes, often without knowing where they would be housed.

Evacuation was a precaution rather than a response to an attack already under way in Britain. Parents had to choose whether to send children away before anyone knew how quickly bombing might start or how long separation would last.

Blackout restrictions changed streets and homes as windows were covered and outdoor lighting was reduced. Air-raid wardens, firefighters, medical services and volunteers prepared for casualties. Barrage balloons and anti-aircraft defences became visible signs that civilian areas were being placed on a wartime footing.

The expected immediate bombing offensive did not arrive on the scale many had feared. Britain instead entered the quieter opening period later called the “Phoney War,” although fighting in Poland was already intense and preparations at home continued.

The next major change came on 17 September

Britain’s declaration did not prevent Poland’s rapid military defeat. German forces continued advancing from the west while Britain and France remained unable to provide immediate, decisive land support inside Poland.

The next major dated milestone came on 17 September 1939, when Soviet forces entered eastern Poland. The intervention deepened Poland’s partition while Britain remained at war with Germany, demonstrating again that the conflict expanded through separate decisions and attacks rather than one event occurring everywhere at once.

Source: Imperial War Museums