[[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. Hitler, using the 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 fast-moving infantry units then exploit the gap and penetrate deep behind enemy lines. Slower moving foot infantry would follow behind the motorized (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 and middle stages of the war, before the Allies developed effective countermeasures.