As a result of his invasion of Poland in September 1939, Britain and France declared war on Germany. By the summer of 1941, Hitler’s armies had occupied most of Europe. Hitler’s foreign policy is most closely associated with the outbreak of World War II, since his decisions to attack Poland, France, the Soviet Union and others escalated the conflict into a global war.

Encyclopaedia Britannica states that Adolf Hitler "was responsible for starting World War II, which resulted in the deaths of more than 50 million people." It explains that on 1 September 1939 "Germany invaded Poland," and that "Two days later both [Britain and France] declared war on Germany, launching World War II."

Britannica’s general article on World War II notes: "The war in Europe began in earnest on September 1, 1939, when Nazi Germany, under Adolf Hitler, invaded Poland." It further states that Britain and France declared war on Germany on September 3, 1939, following this invasion.

“World War II began in Europe on September 1, 1939, **when Germany invaded Poland**. Great Britain and France responded by declaring war on Germany on September 3.” The article describes the invasion as “attack on Poland by Nazi Germany that **marked the start of World War II**.”

“German troops **invaded Poland on September 1, 1939, triggering World War II**.” It adds: “In response to German aggression, Great Britain and France declared war on Nazi Germany on September 3, 1939.” The article notes that Germany launched “the unprovoked attack at dawn on September 1, 1939.”

In its chronology, the USHMM notes: “World War II in Europe began on **September 1, 1939, when Germany invaded Poland**. Britain and France declared war on Germany on September 3, 1939.” The article attributes the invasion to Nazi Germany under Adolf Hitler and places it within a broader pattern of German territorial expansion.

The USHMM biographical overview of Hitler says: "In particular, it is Hitler’s responsibility for World War II and the Holocaust—the systematic persecution and mass murder of Europe’s Jews—that has made him so infamous." In its broader Hitler entries, the Museum describes him as the leader of Nazi Germany whose policies and decisions led directly to the outbreak of the war.

The USHMM "World War II: Key Dates" timeline records: "September 1, 1939: Germany invades Poland, starting World War II in Europe." It lists this invasion, ordered by Adolf Hitler’s regime, as the initiating event of the European war.

Describing the opening shots, the museum states: “It opened fire at 4:43 a.m., **marking the beginning of World War II**.” The article explains that “With his eastern flank secure, **Hitler ordered his generals to finalize ‘Case White,’ the invasion of Poland**.” It calls the September 1939 campaign “the opening act of World War II.”

The National WWII Museum article states: "Upon achieving power, Hitler smashed the nation’s democratic institutions and transformed Germany into a war state intent on conquering Europe for the benefit of the so-called Aryan race." It adds that "His invasion of Poland on September 1, 1939, triggered the European phase of World War II."

“On the morning of 1 September 1939, **Hitler's voice was heard on the radio**. He claimed that Germany had been attacked by Poland and that they had ‘started to fire back at 5:45 am’. However, his story about the Polish attack was a lie… It gave **Hitler an excuse to attack Poland**.” The page states: “England and France had promised to help Poland in the event of a German attack, and so they declared war on Germany. **The Second World War had begun. Hitler had attacked Poland** because he wanted Germans to live there.”

The defendants, acting in concert with others, formulated and executed a Common Plan or Conspiracy to commit, or which involved, Crimes against Peace. These crimes against peace included the planning, preparation, initiation, and waging of wars of aggression, in violation of international treaties, agreements, and assurances. The aggressive wars were directed against, among others, Poland (1 September 1939), the United Kingdom, France, and other countries, and were planned and launched under the leadership of Adolf Hitler and his closest associates.

This educational resource explains that "From 1935 onwards, Germany had actively pursued an aggressive foreign policy" including rearmament and occupations, and that by "pursuing aggressive expansionism, Germany’s actions made a major European war more likely." It notes: "The Nazis invaded Poland on 1 September 1939" and describes this as the point at which the war began, while also stressing that "Germany’s aggressive foreign policy was not the sole cause of the Second World War, but it was a large contributing factor."

History.com’s overview states: "The instability created in Europe by the First World War (1914-18) set the stage for another international conflict—World War II—which broke out two decades later and would prove even more devastating." It continues: "The war began when Germany invaded Poland on September 1, 1939, prompting Britain and France to declare war on Germany."

The educational site notes: “From 1935 onwards, **Germany had actively pursued an aggressive foreign policy**… before eventually **invading Poland in 1939**.” It adds that “The Nazis invaded Poland on **1 September 1939**… On 17 September, the Soviet Union joined forces with Germany and invaded Poland.” It comments that “Germany’s aggressive foreign policy was not the sole cause of the Second World War, but it was a large contributing factor,” and that Allied failures to work together “were a contributing factor in the outbreak of the Second World War.”

BBC Bitesize explains: “On **1 September 1939 Hitler invaded Poland**. Britain and France declared war on Germany on **3 September 1939**.” It also notes that “Hitler’s aggressive foreign policy was a major cause of the Second World War,” listing actions such as remilitarising the Rhineland, the Anschluss with Austria, the occupation of Czechoslovakia, and finally the invasion of Poland.

BBC News, in an explainer on the war’s origins, writes: "The war began when Nazi Germany, led by Adolf Hitler, invaded Poland on 1 September 1939." It further explains that Britain and France had guaranteed Poland’s borders and so they declared war on Germany on 3 September 1939, marking the start of World War Two in Europe.

France 24’s historical piece on the anniversary reports: "On September 1, 1939, Nazi Germany invaded Poland, starting World War II." The article emphasizes that the invasion was ordered by Adolf Hitler and that it triggered declarations of war by Britain and France two days later.

The Adolf Hitler entry notes: "His aggressive, expansionist foreign policy is considered the primary cause of World War II in Europe." It also records that "On 1 September 1939, Hitler oversaw the German invasion of Poland, the event that is generally regarded as the start of World War II."

The World War II article states: "Most historians date the beginning of the war to 1 September 1939, the day of the German invasion of Poland." It describes Nazi Germany, led by Adolf Hitler, as pursuing a program of expansion and records that Britain and France declared war on Germany on 3 September 1939 in response.

In response to a question about the timeline, the historian states: "Most historians and scholars agree the approximate start of the Second World War is Hitler's invasion of Poland on September 1st, 1939." He explains that this attack led Britain and France to declare war on Germany, marking the beginning of the wider conflict in Europe.

This university history page describes Adolf Hitler as "unquestionably the central figure" in the Nazi regime and the Holocaust. While focused on his rise, it emphasizes that his political success and dictatorship made the Nazi program, including war and genocide, possible, underscoring his centrality to Germany’s path to World War II even though it does not frame him as the sole cause of the conflict.

The Nuremberg Tribunal charter, which set the framework for prosecuting Nazi leaders after the war, lists among the crimes under its jurisdiction "Crimes against Peace" defined as "planning, preparation, initiation or waging of a war of aggression." In subsequent Nuremberg judgments (referenced within the same collection), leading Nazi officials, including Hermann Göring and others close to Hitler, were convicted for participating in a common plan to wage aggressive war, with the German invasions of Poland and other countries cited as key acts. This legal framing treated the Nazi leadership under Hitler as responsible for initiating aggressive war, though it did not attribute legal responsibility only to Hitler personally.

The article notes that after the Nazi seizure of power, "Hitler’s diplomatic tactics were to make seemingly reasonable demands, threaten war if they were not met, and claim he had no further territorial ambitions" while pursuing expansion in violation of Versailles. It states that World War II in Europe "began on 1 September 1939, when Germany invaded Poland," and that the invasion followed Hitler’s decision-making, including the earlier remilitarisation, annexations, and the Nazi–Soviet Pact.

As Führer and Reich Chancellor of Nazi Germany, Adolf Hitler held ultimate decision-making authority over German foreign and military policy in 1939. Historians generally agree that the decision to invade Poland on 1 September 1939 was taken under his leadership and reflected his expansionist aims for Lebensraum and revision of the post–World War I settlement.

This educational video for children describes Adolf Hitler as "infamous for initiating World War II by invading Poland in 1939." It explains that his push for expansion of German territory "led to the outbreak of World War II in 1939 when Germany invaded Poland."