WW2 History defines that war as beginning in 1939 in Europe with the battles for Poland.
Post-Versailles Poland was a country of some 24 million. Poland was only 75 percent Polish; the rest was Russian or Ukrainian in the east or German in the west. The Polish Corridor (including the port of Danzig) not only was heavily German but separated East Prussia from the Fatherland, a source of tension that Hitler eagerly exploited.
Poland had beaten the Red Army during the Russian civil wars after 1918 and remained impressive enough to motivate France to sign a mutual defense treaty in the 1920s. But if the Poles detested Germany, they detested Russia even more and so rejected French entreaties to permit Soviet troops into Poland if Germany attacked.
By mid-1939 Hitler, deprived of war in Czechoslovakia, was committed to one in Poland. Despite the Franco-Polish treaty and the growing alarm in London, he did not think the West would interfere, particularly after he signed a startling nonaggression and trade pact with Premier Joseph Stalin that included a secret protocol allowing the Soviets to occupy eastern Poland in case of war.
Even so, the Poles did not altogether despair. They believed that France and Britain would eventually respond and that the Polish army could withstand the Wehrmacht for many months, long enough for the West to mobilize and confront Hitler with what he feared most – a two-front war. This proved only partly accurate. When the Germans attacked on September 1, France and Britain, after issuing an ultimatum, did declare war. But they intended less to fight for Poland, which they considered indefensible without Russian involvement, than to signal to Hitler that they would fight him at some point.
The Poles moreover did not hold out, largely because the Germans fought a war that emphasized surprise and velocity as well as firepower. However, WW2 History has begun…