Blitzkrieg!

German plane bombing Warsaw, 1939.
(Public domain)
It is called "lightning war."  And on September 1, 1939, Hitler unleashed the full power of his military on Poland.

Like in the invasion of Manchuria, the incident that led to the invasion was made up by the German military. Accusing Poland of attacking a radio tower, the invasion began in the morning with deadly bombing runs of Polish cities and towns.  Fast moving tanks poured across the border.

Although Poland had been promised protection, no one came to her aid.  The attack on Poland resulted in both Great Britain and France declaring war on Germany. Within weeks, Poland was no longer an independent country, but had been taken over by Germany (and the USSR claimed parts of eastern Poland as well).  Within weeks, some 20,000 Polish citizens were killed at mass extermination sites scattered throughout Poland.  Civilian losses may have totaled more than 300,000 during the invasion and subsequent occupation.

This incident is traditionally viewed as the starting point of World War II.