Many people think of World War II as a long conflict filled with famous battles like D-Day or Stalingrad. But everything that followed—every alliance, every strategy, every turning point—was shaped by what happened in 1939.
This was not just the beginning of war. It was the result of years of tension, failed diplomacy, and calculated risks. To truly understand the war, you have to start here.
If you're building a broader understanding, it helps to connect this year with the full timeline of events on WW2 key events timeline and the background causes explained in Treaty of Versailles impact on WWII.
By early 1939, Europe was already unstable. Germany had expanded aggressively under Adolf Hitler, annexing Austria and taking control of Czechoslovakia without major resistance from Britain or France.
This policy of avoiding conflict—often called appeasement—gave Germany confidence that further expansion might go unchallenged.
In August 1939, Germany and the Soviet Union signed the Molotov–Ribbentrop Pact. On the surface, it was a non-aggression treaty. In reality, it included secret agreements dividing Eastern Europe into spheres of influence.
This meant Poland would be split between the two powers—an agreement that made invasion almost inevitable.
Germany launched its attack using Blitzkrieg, or "lightning war." This strategy combined fast-moving tanks, coordinated air strikes, and rapid infantry movement.
Poland was unprepared for this speed and coordination.
Despite having a large army, Poland lacked modern equipment and coordinated defense strategies. Within weeks, German forces overwhelmed Polish resistance.
Then, on September 17, the Soviet Union invaded from the east, sealing Poland’s fate.
On September 3, 1939, Britain and France declared war on Germany. This officially turned a regional conflict into a global one.
However, despite the declaration, there was little immediate fighting in Western Europe. This strange period became known as the "Phoney War."
From late 1939 into early 1940, Western Europe experienced a pause in major military action. Armies mobilized, but few direct confrontations occurred.
This gave Germany time to plan its next moves—and gave Britain and France a false sense of security.
To understand how this pause ended, continue with WW2 events in 1940.
Key Concept: The war did not start suddenly—it was the result of calculated decisions and systemic failures.
For a broader perspective, explore WW2 timeline facts or return to the main hub at WW2 Homework Help.
Most explanations stop at “Germany invaded Poland.” But that’s only part of the story.
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World War II began in 1939 primarily because of Germany’s invasion of Poland. However, the deeper causes go back much further. The Treaty of Versailles placed heavy economic and territorial restrictions on Germany after World War I, creating resentment and instability. Adolf Hitler used this dissatisfaction to justify expansion. Additionally, Britain and France followed a policy of avoiding conflict, which allowed Germany to grow stronger without consequences. The final trigger came when Germany invaded Poland, and Britain and France responded by declaring war. Without this chain of events, the war might have been delayed or taken a different form.
The Molotov–Ribbentrop Pact was a non-aggression treaty between Nazi Germany and the Soviet Union signed in August 1939. Publicly, it promised that neither country would attack the other. Privately, it included secret agreements to divide Eastern Europe into spheres of influence. This allowed Germany to invade Poland without fear of Soviet intervention and enabled the Soviet Union to take control of eastern Poland and other regions. The pact shocked the world because the two nations had opposing ideologies, but it demonstrated how strategic interests can outweigh political differences in international relations.
Blitzkrieg, meaning “lightning war,” was a military strategy used by Germany that focused on speed, coordination, and surprise. Instead of slow, traditional warfare, Blitzkrieg used tanks, aircraft, and infantry in a coordinated attack to break through enemy defenses quickly. Communication systems were targeted first, preventing organized resistance. This approach was highly effective in Poland because it overwhelmed defenders before they could respond. The success of Blitzkrieg in 1939 influenced German strategies in later campaigns, especially in Western Europe in 1940.
Britain and France had the opportunity to confront Germany before 1939 but chose not to. This was largely due to the trauma of World War I, which made leaders hesitant to enter another conflict. They believed that allowing Germany limited expansion might prevent a larger war. This approach, known as appeasement, ultimately failed because it encouraged Germany to continue its aggressive policies. By the time Germany invaded Poland, it was too late to avoid war, and Britain and France were forced to act.
The Phoney War refers to the period between late 1939 and early 1940 when Britain and France were officially at war with Germany but engaged in very little actual fighting. Despite large troop mobilizations and defensive preparations, neither side launched major attacks. This period created a false sense of calm and allowed Germany to prepare for future offensives. It ended in 1940 when Germany launched attacks on Western Europe, leading to rapid and dramatic changes in the war.
While later years of World War II included major battles and turning points, 1939 is crucial because it set everything in motion. The alliances, strategies, and political decisions made during this year shaped the entire conflict. Without understanding 1939, it becomes difficult to fully grasp why later events happened the way they did. This year established the structure of the war and determined its early direction.