[[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 GermansHitler, using the supremacy supposed attacks by the Poles as his reasoning, invaded Poland. The major tactical innovation of the war was the use of combined arms warfare, typified by the German doctrine of [[blitzkrieg]]. In this style of warfare armor, infantry, artillery and air power (see [[Luftwaffe]] ) all coordinate to achieve overwhelming superiority at point on the enemy lines. Armor and lightning attacks by armored fast-moving infantry units then exploit the gap and penetrate deep behind enemy lines. Slower moving foot infantry would follow behind the motorized units(later mechanised) and armored forces, securing their flanks in order to prevent encirclement. The objective is to cause a widespread collapse of the enemy's ability to fight. It was particularly effective during the early stages of the war, swept aside before the partially mobilized Polish armyAllies developed effective countermeasures.
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.