What provoked England and France to declare war on Germany in 1939?

England and France declared war on Germany in 1939 primarily due to Germany's invasion of Poland on September 1, which violated the Treaty of Versailles and threatened the balance of power in Europe. Both countries had pledged to support Poland in the event of an attack, and when Germany refused to withdraw its troops, they felt compelled to act. The declaration of war on September 3 marked a significant escalation in the conflict, as it was clear that Germany's aggressive expansionist policies posed a direct threat to European stability.