Why did Germany’s invasion of Poland begin World War II?

Germany’s invasion of Poland on September 1, 1939, triggered World War II because it directly violated the sovereignty of a nation that had received formal military guarantees from Great Britain and France.12

Prior to the invasion, Adolf Hitler sought to neutralize the risk of a two-front war by signing the German-Soviet Nonaggression Pact, which included a secret protocol to partition Poland between Germany and the Soviet Union. Despite this, Britain and Poland had signed a treaty of mutual assistance on August 25, 1939, which obligated the British to intervene if Poland were attacked.132

When Hitler ignored diplomatic efforts to restrain his aggression and launched the invasion, Britain and France issued ultimatums demanding a withdrawal. Following Germany’s refusal to comply, both nations declared war on Germany on September 3, 1939, marking the formal beginning of the global conflict.21