The final days of peace

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The Final Days of Peace: Why Britain Declared War on Germany in September 1939

Britain’s declaration of war on Germany on Sunday, 3 September 1939, was not an impulsive decision. It followed months of increasingly serious confrontations and, ultimately, a desperate 48-hour diplomatic crisis.

What makes those final days particularly interesting is that Neville Chamberlain was still trying to avoid war even after Germany had invaded Poland.

There were three critical stages: the collapse of Britain’s trust in Hitler, Britain’s commitment to defend Poland, and the diplomatic exchanges of 1–3 September.

1. The decisions that brought Britain to the brink

30 Sept 1938

The Munich Agreement

Chamberlain, Hitler, Mussolini and French Prime Minister Édouard Daladier agree to allow Germany to annex the Sudetenland, part of Czechoslovakia.

Chamberlain returns to Britain believing he has secured peace. Hitler claims that he has no further territorial ambitions in Europe.

15 March 1939

Germany occupies the Czech lands

German forces enter Prague, destroying the remaining independent Czech state.

This is a turning point. Hitler is no longer merely demanding territories with substantial German-speaking populations. Chamberlain’s strategy of negotiating concessions has suffered a decisive setback.

31 March 1939

Britain guarantees Poland’s independence

Chamberlain announces in Parliament that Britain will support Poland if its independence is threatened and Poland resists.

This fundamentally changes Britain’s position. Further German aggression against Poland now risks triggering British involvement.

23 Aug 1939

Germany signs a non-aggression pact with the Soviet Union

Hitler and Stalin’s governments agree not to attack each other. Secret arrangements divide parts of Eastern Europe, including Poland, into spheres of influence.

Hitler can now attack Poland with less immediate risk of fighting the Soviet Union.

25 Aug 1939

Britain signs a formal mutual assistance agreement with Poland. Britain reinforces its earlier guarantee. The agreement makes clear that aggression against Poland can bring British assistance.

Hitler nevertheless continues preparations for invasion.

By the end of August, the British government had reached a difficult position: it still wanted to avoid another European war, but withdrawing its promise to Poland would undermine its attempt to deter further German expansion.

2. The final 48 hours: 1–3 September 1939

Friday, 1 September — Germany invades Poland

At approximately 4:45 am, German forces cross the Polish frontier.

Hitler claims that Germany is responding to Polish aggression. This claim is false; Germany has initiated the attack.

Poland appeals to Britain and France to honour their commitments.

Throughout the day, British officials exchange urgent diplomatic messages with Berlin, Paris, Rome and Warsaw.

At approximately 9 pm, British Ambassador Sir Nevile Henderson delivers a formal warning to German Foreign Minister Joachim von Ribbentrop, demanding that Germany stop its attacks and withdraw from Poland.

No satisfactory answer is received.

Saturday, 2 September — Chamberlain faces pressure at home

This is perhaps the most revealing day in the entire story.

Britain has still not declared war, despite the German invasion.

Chamberlain consults the French government about issuing an ultimatum.
Mussolini proposes an international peace conference.
British ministers insist that German troops must withdraw before meaningful negotiations can begin.
Members of Parliament become increasingly concerned by the delay.

That evening, Chamberlain addresses the House of Commons but announces no definite deadline for Germany.

Arthur Greenwood, acting leader of the Labour opposition, challenges the government’s hesitation.

Conservative MP Leo Amery famously calls out:

Speak for England, Arthur!

There is now significant pressure from both opposition MPs and members of Chamberlain’s own party to act. hansardsociety.org.uk

Sunday, 3 September — The final ultimatum

9:15 am

British Ambassador Henderson meets Ribbentrop in Berlin and delivers Britain’s final ultimatum.

10:30 am

The British War Cabinet meets as the deadline approaches.

11:00 am

The ultimatum expires. Germany has not agreed to withdraw from Poland. A state of war now exists between Britain and Germany.

11:15 am

Chamberlain broadcasts from 10 Downing Street, informing Britain that the country is at war.

11:30 am

Air-raid sirens sound in London. The alarm proves to be false.

12:06 pm

Chamberlain addresses the House of Commons, explaining the government’s decision.

5:00 pm

France’s ultimatum expires, and France enters the war against Germany.

3. Did Chamberlain actually have a choice?

In the narrowest sense, yes. Britain could have abandoned its guarantee to Poland. But that would have meant accepting another German conquest after repeated efforts to contain Hitler through diplomacy.

Chamberlain faced several competing considerations: the obligation to Poland, Britain’s international credibility, the military danger of confronting Germany, public fears about another world war and the rapidly diminishing prospect of a negotiated settlement.

It is important to recognise that Britain did not declare war simply because Germany invaded Poland. The government spent almost two further days seeking a German withdrawal before allowing its ultimatum to expire.

The immediate decision was made by Chamberlain’s government, not by a public referendum or a parliamentary vote specifically authorising the declaration.

4. The historical documents

Two original records are especially useful for understanding what happened inside the British government.

House of Commons debate — 2 September 1939⁠

The parliamentary arguments and political pressure on Chamberlain the evening before war was declared.

Chamberlain’s statement to Parliament — 3 September 1939⁠

His account of the ultimatum, Germany’s failure to respond satisfactorily and Britain’s entry into the war.

The particularly interesting historical question is whether Britain might have avoided war altogether had Chamberlain confronted Hitler over Czechoslovakia in 1938, rather than waiting until Poland was invaded in 1939.

That debate remains central to understanding how Europe descended into the Second World War.