On September 1st 1939 Germany launched an attack against Poland.
In accordance with a secret agreement with the Germans the Soviet Union invaded eastern Poland on September 17th.
After defeating Polish forces at the Battle of Bzura the Poles retired to a fixed line of defences near the Romanian border and waited for the Western Allies to intervene.
The Western allies, Britain and France duly declared war in support of Poland but were in not position to provide direct support. By October 6th Poland, after the Polish defeat at the Battle of Bock, was controlled jointly by Nazi Germany and the Soviet Union. On October 8th Germany formally annexed western Poland. Russia follwed by annexing eastern Poland.
The key to the swiftness of the German campaign was in the aggressive use of mechanised forces to punch holes through the enemy defences. Weak spots would be ‘softened up’ using artillery and bombers. Fast armoured vehicles would attack these ‘soft’ points and then move around the enemy rear cutting off communications and supplies, forcing the opposition to either withdraw or surrender. This type of fast offensive action would eventually become referred to as Blitzkrieg, the German for 'Lightning War'.
Radio communications had been established throughout the German command structure and was used to co-ordinate the efforts of all arms against the enemy strong points.
Dive bombers were available on demand to support ground attacks through the precision bombing of enemy troops and installations.
Reconnaissance was essential to find out where the enemy line was weakest. Armoured cars were ideal for this task.
The French and British possessed some mechanised units but they were subordinate to the slow moving infantry, lacked effective communications and were wasted in poorly co-ordinated counter attacks.
Vehicles capable of carrying supplies and weapons were an important part of any mechanised army. Trucks and tracked vehicles were used to provide supplies to the front line troops.
On September 3rd 1939 both Britain and France declared war on Germany after their demands for the withdrawal of German troops from Poland had not been met.
There followed a brief period whilst Germany concentrated her armed forces on the borders with France and the Low Countries and decided how to continue.
The German armoured divisions were finally unleashed on May 10th 1940, making a diversionary attack through Holland to draw the Allies north.
This was followed with an attack by mechanised troops from Belgium, through the Ardenne Froest, towards the English Channel, cutting off the British and French troops in Belgium.
Medium tanks, armed with large calibre howitzers, were used to support the infantry. Lighter, faster main battle tanks armed with smeller high velocity weapons were intended to exploit any breach in the enemy defensive line and drive at the enemy rear to keep them off balance.
Tanks were not expected to engage in battles with other tanks. Tanks were supposed to be dealt with by specialised troops using high velocity anti-tank guns with special ammunition developed specifically to deal with armour. German tactical doctrine was defensive, they would attack, form a defensive perimeter and then wait for the expected counter-attack, which would typically be met with machine gun fire, artillery and anti-tank weapons.
Paratroops were dropped behind enemy lines to secure strategic objectives like bridges and fortresses. The Belgian fort of Eben Emael, located near the Albert Canal on the Belgian-Dutch border was effectively taken by German airborne troops delivered to the target by glider.
Many Allied frontline airfields were overrun with the loss of vital spares and support equipment.
Swift agressive action by the Luftwaffe caught many Allied aircraft on the ground in surprise attacks, preventing the Allies from challenging German air superiority.
Fighters were used to secure air superiority and defend bombers from other fighters. Bombers were used tactically to attack ground targets in support of the army.
After the French surrendered on June 22nd 1940 a lot of the captured equipment was pressed into service with the German forces. At the time much of this captured equipment would have been superior to German equipment.
The British Expeditionary Force left most of its heavy equipment behind on the beaches during the evacuation from Dunkirk.