Contrary to the established terminology, the “Second Front” during WW2 was opened by Hitler on June 22, 1941. This was the day when Nazi Germany attacked their former ally, the Soviet Union, in the hope of a quick victory. In the meantime, even after the defeat of France in 1940, there were continuing military clashes in the west: in Africa, in the skies above Britain and around the Atlantic ocean.
Why did Hitler found himself fighting in the west long before the war with the Soviet Union, in spite of his old program of expansion to the East of Europe?
Imminence of threat
France was considered much more dangerous than anyone else. Militarily, it had the largest land force (Hitler didn’t know about the scale of Soviet military build-up), was closer to the German heartland than the USSR, and enjoyed strategic support from the UK.
As an ally of Poland, France find itself obliged to declare a war on Germany in September ’39. Going east with such an enemy force in the back in the state of war for Hitler would be an unacceptable risk. He needed to finish off the French before everything else.
Economic considerations
Hitler had an unsustainable budget deficit throughout his entire rule and started WWII with a debt of 40 billion Reichsmarks against a GDP of 30 billion Reichsmarks. Back in the 1930s, if you had the choice of Bolshevik Russia versus France for where to go for good old sacking and plunder, the choice was obvious.
Imperatives of national psyche
France was seen as the evil force in the recent German history. First as primary driving force maintaining German nations divided and in conflict with each other during the 17th and 18th century. Then, the brutal robber of German land, pride and wealth as the victor in WWI.
Strategic realities
France is much more suited for “blitzkrieg”, the cherished baby of Hitler´s geopolitical brain, than Russian plains where nature and the soil itself rejects paved roads. Besides, before engaging the USSR, he needed to take Poland, and Poland was tied with France.
Fateful error
The West had to be crushed first, before Hitler could open the second front in the east. With Britain defeated, or with a simple truce across the Channel, Hitler could have avoided dragging the US into the fray. Failure to do so led to his ultimate defeat in WW2.
Paradoxically, this also caused the annihilation of almost the entire Red Army in the battles of 1941. Stalin gambled that Hitler wouldn’t attack before striking a deal with Churchill — and almost lost the war.