World War II began when Nazi Germany under Adolf Hitler invaded Poland on September 1st, 1939. This invasion was under the false pretext that Poland had orchestrated a series of sabotage operations against German targets along the border. The reality is actually the opposite, German commandos had staged pretend raids so as to give a casus belli for their invasion of Poland. The Germans, using the supremacy of the Luftwaffe and lightning attacks by armored and motorized units, swept aside the partially mobilized Polish army.
On September 17th, the Soviet Union invaded Poland. Stalin had secured a ceasefire with Japan over the Khalkhin Gol incidents, allowing him to follow up on his promise to invade Poland from the east, as per the Molotov-Ribbentrop Pact. The combined weight of German and Soviet forces sealed the inevitable. Poland surrendered on September 27th, following the fall of Warsaw. Remaining pockets of resistance were crushed and surrendered on October 6th.
Britain and France declared war on Germany two days after the invasion of Poland began. The Dominions of the British Commonwealth followed shortly after. Polish resistance would soon begin, and would continue throughout the entire war. Poland was divided between Germany and the USSR. Germany acquired the western regions, approximately 48.4% of the country.[1] The Soviet Union acquired the rest. About 270 sq. miles of land was given to Germany's puppet state Slovakia.