Europe at the end of WWI was dire. The Treaty of Versailles which officially ended WWI imposed harsh conditions on the defeated nation of Germany. The Germans were limited to a 100,00 man army which they used as a training organization for an envisioned German Army of the future. The Germans learned the lessons of the First World War well.
They realized the potential of the tank and the airplane to expand the battlefield and to limit the possibility of a stalemated front. The Germans would develop military doctrine to fully employ the potential of these new weapons to great effect as we will see beginning in 1939.
We will also read about the state of military affairs and thinking in the western powers of Britain and France. France more than any of the western democracies was reputed to have the greatest army in the world. But by the late 1930s the French army was a shadow of the great army that took the field in 1914.
The French having suffered tremendous losses in WWI abandon the strategy of the offensive in favor of a defensive strategy anticipating a future conflict similar to the one they experienced in WWI. This defensive state of mind is best seen in the line of defensive works they built in the 1930s know as the Maginot Line.
The British along with the French entered a period of stagnation in military thought. Although General Fuller and Liddell Hart advocated tank warfare the General Staff stuck to infantry based strategies with tanks in a supporting role only. The British were also exhausted by the effort expended in WWI both politically, and morally.
Adolph Hitler came to power in 1932 to right the wrongs of the Versailles treaty. He managed to bully his way into Czechoslovakia and Austria. He then turned his attention to Poland to regain lost German territory and begin the great eastward expansion of the Reich.
In August 1939 Hitler stunned the world by signing the Russo-German Nonaggression Pact with Stalin. This pact between deadly enemies would assure him a free hand in Poland. On 1 September 1939 the new German Army invaded Poland and several days later Britain and France fulfilled their treaty obligation to Poland by declaring war on Germany.
The Polish campaign stunned the military world. For the first time massed formations of tanks tore through infantry formations and fixed defenses isolating pockets of resistance and in conjunction with ground support aircraft creating havoc behind the Polish lines.
Soon many Polish units were isolated and forced to surrender or be destroyed. The Polish campaign ended in a complete German victory and proved the German concept of Blitzkrieg warfare was the war fighting method of the future.
After the Polish campaign an interlude of inactivity ensued. The most notable events of the winter of 1939-40 were the Russian attempts to invade Finland and the movement of the German forces west to face the armies of France and Britain.
The Winter War between Russia and Finland showed the world the true state of the Soviet army after the Stalinist purges of the officer corps in the late 1930s. The Finns badly outnumbered managed to outfight and rout larger Russian formations.
The Finns were supported by Britain, France and Germany against the Russians. Resistance ended with a massive Russian offensive that the badly outnumbered Finns could not contain. The campaign forced much needed reforms on the Russian Army which were implemented in 1940 and 1941.