Fall Weiss

Fall Weiss was the German strategic plan for the invasion of Poland. The German military High Command finalized its operational orders on 15 June 1939 and the invasion commenced on 1 September, precipitating World War II.

Background

The origins of the plan went back to 1928, when Werner von Fritsch started working on it. Fall Weiss was developed primarily by Günther Blumentritt and Erich von Manstein while the two were serving as staff officers under General Gerd von Rundstedt with Army Group South in Silesia.

The plan called for a start of hostilities before the declaration of war.
Wehrmacht units were to invade Poland from three directions:

A primary attack from the German mainland across the western border of Poland.
A secondary attack from the north, from the exclave of East Prussia.
A tertiary attack by German and allied Slovak units across the border of Slovakia.

All three assaults were to converge on Warsaw while the main Polish army was to be encircled and destroyed west of the Vistula River.
The plan was initiated on 1 September 1939 and was the first European military operation of World War II.