Germany invaded Poland
On September 1, 1939, Germany concentrated 1.6 million men, with the cooperation of more than 2,000 aircraft and 2,800 tanks, and launched a major offensive against Poland. On September 3, Britain and France declared war on Germany, and World War II broke out in full swing.
Poland put up stubborn resistance, but the Polish army was poorly deployed, scattered on a long national border, lacking defense in depth; the Polish army was backward in equipment compared to the German army, and the number and quality of tanks and aircraft could not be compared with the German army.
The German army used the new tactic of "blitzkrieg" in the war, first using the air force to violently bombard the enemy's important strategic facilities and communication centers, seizing the air supremacy on the battlefield, and then concentrating on the use of armored forces, quickly breaking through the enemy's defense line, encircling the enemy's rear, and quickly eliminating the enemy's living forces.
After Britain and France declared war on Germany, they did not launch a large-scale offensive against the Germans on the Western Front. Poland was alone and outnumbered. Warsaw fell at the end of September. The Soviet Union seized the opportunity to expand its territory westward and occupy eastern Poland.
After occupying Poland, Hitler randomly shifted the center of gravity of the war to the Western Front, against Germany's old enemy, France.