Historical eventSeptember 171939

World War II

Second World War: The Royal Navy lost its first warship in the war when German submarine U-29 torpedoed and sank HMS Courageous.

Everything on September 17Original source

In context

When
Sunday, September 17 193987 years ago
Category
Historical eventA dated happening — a battle, treaty, launch, discovery, disaster or decision — recorded on this calendar day.
Era
20th century · 1930s
Recorded from
Wikipedia “On this day” feed

At a glance

Dates: From 1939.

What came beforeAI-assisted

World War I (1914–1918) left Germany economically devastated and humiliated by the Treaty of Versailles, which imposed harsh reparations and territorial losses. The global economic depression of the 1930s further destabilized Europe, creating conditions for the rise of fascist and militaristic regimes.

Japan invaded Manchuria in 1931 and China in 1937, while Italy invaded Ethiopia in 1935 and Albania in 1939. Germany, under Adolf Hitler, remilitarized the Rhineland (1936), annexed Austria (1938), and seized Czechoslovakia (1938–1939), all with minimal international resistance.

Catalyst / how it startedAI-assisted

Germany invaded Poland on September 1, 1939, using blitzkrieg (lightning war) tactics to rapidly overwhelm the country. Britain and France declared war on Germany two days later, on September 3, 1939, fulfilling their guarantee to defend Polish independence.

On September 17, 1939, the Soviet Union invaded Poland from the east, as secretly agreed in the Molotov-Ribbentrop Pact (Nazi-Soviet Non-Aggression Pact) signed in August 1939. Poland was defeated and partitioned between Germany and the Soviet Union by early October.

Who was involvedAI-assisted

The Axis powers were led by Nazi Germany (Adolf Hitler), Imperial Japan (Emperor Hirohito), and Fascist Italy (Benito Mussolini). The Soviet Union initially remained neutral under the Nazi-Soviet Pact but joined the Allies after Germany's invasion in June 1941.

The Allies included Britain and France from 1939, joined by the Soviet Union in 1941 and the United States in 1941 after the attack on Pearl Harbor. China had been fighting Japan since 1937 and formally joined the Allies in 1941.

TimelineAI-assisted

September 1, 1939: Germany invades Poland. September 3, 1939: Britain and France declare war on Germany. September 17, 1939: Soviet Union invades Poland from the east.

June 22, 1941: Germany invades the Soviet Union. December 7, 1941: Japan attacks Pearl Harbor; the United States enters the war. May 7–8, 1945: Germany surrenders unconditionally. August 6–9, 1945: Atomic bombs dropped on Hiroshima and Nagasaki. September 2, 1945: Japan formally surrenders.

What happened

World War II, or the Second World War, was a global conflict between two coalitions: the Allies and the Axis powers. Nearly all of the world's countries participated, with many engaging in total war on an unprecedented scale. World War II was the deadliest conflict in history, causing the deaths of 60 to 75 million people, a majority of whom were civilians. Millions died as a result of massacres, starvation, disease, and genocides including the Holocaust. After the Allied victory, Germany, Austria, Japan,

September 17 through the years

Other entries on the same calendar day, in order.

  1. 1939World War II
  2. 2006Őszöd speech
  3. 2006Fourpeaked Mountain
  4. 2008Al-Qaeda in Yemen
  5. 2011Adbusters
  6. 2011Occupy Wall Street
  7. 2013Grand Theft Auto V
  8. 20162016 New York and New Jersey bombings
  9. 2018Israeli Air Force
  10. 2018Syria missile strikes (September 2018)
  11. 2025Strategic Mutual Defence Agreement

Also in 1939

  • September 9World War IIWorld War II: About 3,000 Polish Army troops began a nearly month-long defence of the Hel Peninsula during the German invasion of Poland.
  • October 20Pope Pius XIIPope Pius XII (pictured) published his first encyclical, Summi Pontificatus, which critiqued ideologies such as racism, cultural superior…
  • October 14World War IISecond World War: The German submarine U-47 torpedoed and sank the Royal Navy battleship HMS Royal Oak while the latter was anchored at S…
  • December 22All-India Muslim LeagueMembers of the All-India Muslim League observed a "Day of Deliverance" to celebrate the resignations of members of the Indian National Co…
  • May 22Pact of SteelGermany and Italy signed the Pact of Steel, a military and political alliance.
  • January 7Marguerite PereyFrench physicist Marguerite Perey identified francium, the last element to be discovered in nature rather than by synthesis.
  • November 26Red ArmyThe Soviet Red Army shelled the village of Mainila and then claimed that the fire originated from Finland, giving them a casus belli to l…
  • January 14Queen Maud LandNorway claimed Queen Maud Land, a 2.7-million km2 (1.0-million sq mi) region of Antarctica, as a dependent territory.

Everything we record for 1939 →

Sources

Cite this page

“World War II”, What's On This Date, https://whatsonthisdate.com/app/event/world-war-ii-1939-september-17, accessed October 26, 2026.

Cite the underlying source as well — the source links above go to the record this entry is adapted from.

Questions about this entry

When did World War II happen?

September 17 1939 — 87 years ago, on a Sunday.

Where does this text come from?

Wikipedia “On this day” feed. Summaries are short adaptations that link back to the source, not reprints. Panels marked AI-assisted were filled where the sources left a heading empty.

How do I cite this page?

Use the ready-made citation near the sources at the bottom of the page, and follow the source link to cite the underlying record as well.

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