On September 1, 1939, German forces under Adolf Hitler invaded Poland, an event that is widely regarded as the start of World War II. Using a 'Blitzkrieg' (lightning war) strategy that coordinated air power and rapid armored movements, Germany quickly overwhelmed Polish defenses. Two days later, honoring their defensive treaties, Great Britain and France declared war on Germany. Following a secret agreement in the Molotov-Ribbentrop Pact, the Soviet Union also invaded Poland from the east on September 17. The conquest of Poland led to the brutal occupation of the country and the establishment of the first ghettos for Jewish citizens. This event shattered the policy of 'appeasement' previously held by European powers and initiated a six-year global conflict that would eventually involve almost every major nation in the world.