The Nazi invasion of Poland started World War II primarily because it was a direct act of aggression that violated the treaties and alliances of the time, prompting Britain and France to declare war on Germany in defense of Poland. Hitler's invasion on September 1, 1939, followed a secret Nazi-Soviet pact dividing Poland between the two powers and was driven by Nazi aims of territorial expansion, securing resources, and ideological goals like Lebensraum and ethnic domination. The invasion was the immediate trigger for Britain and France, who had guaranteed Poland's independence, to declare war on Germany, marking the official start of World War II.

Reasons the Invasion Started WWII

• Nazi Germany invaded Poland with the goal of disestablishing Poland as a sovereign state and expanding living space eastwards (Lebensraum).
• The invasion followed the secret Molotov-Ribbentrop Pact with the Soviet Union, agreeing to divide Poland between Germany and the USSR.
• Britain and France had military alliance guarantees with Poland; Germany’s attack forced them to declare war on Germany on September 3, 1939.
• Hitler used staged provocations (like the Gleiwitz incident) to justify the invasion as a defensive action.
• The attack was a blatant violation of international agreements and norms, leaving Britain and France no choice but to respond with war declarations to contain Nazi aggression.

Context of the Invasion

• Germany feared encirclement and sought to revise territorial losses from the Treaty of Versailles.
• Nazi ideology demanded the conquest and ethnic cleansing of Slavic populations, including Poles.
• The invasion was a large-scale, surprise Blitzkrieg assault with overwhelming military force.
• The Soviet invasion from the east on September 17 compounded Poland’s defeat and the division of its territory.

Thus, the Nazi invasion of Poland is considered the start of WWII because it triggered the first declarations of war by major Western powers against Germany due to their treaty obligations and the aggressive threat posed by Nazi expansionism.