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How did the invasion of Poland lead Britain and France to declare war on Germany?

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Nazi SS troops march in formation carrying banners with swastikas and eagles at the 1933 Party Day rally in Nuremberg, Germany, with trees in the background. — Third Reich A Nazi Party rally at Nürnberg, Germany, in 1933.

The invasion of Poland led Britain and France to declare war on Germany because both nations had formally committed to defending Polish sovereignty against aggression.12

Following the German occupation of Czechoslovakia in March 1939, Britain and France abandoned their policy of appeasement and issued a military guarantee to Poland. When Adolf Hitler, emboldened by the German-Soviet Nonaggression Pact, ordered the invasion of Poland on September 1, 1939, he mistakenly believed that the Western powers would withdraw their support.3421

Instead, Britain and France honored their treaty obligations. After delivering an ultimatum demanding the immediate withdrawal of German forces from Polish territory, both nations declared war on Germany on September 3, 1939. This decision marked the beginning of World War II, as the Allies refused to allow further unprovoked expansion by the Third Reich through force.1254