The Ninth Corps was activated on October 24th, 1940,
with Headquarters at Fort Lewis, Washington. Following maneuvers
at Hunter Ligget Military Reservation in California, the Ninth
returned to Fort Lewis where it was ordered to place Operation
Plan Rainbow 5 into effect on December 7th, 1941, after the
Japanese attack on Pearl Harbor.

The shoulder patch worn by the IX Corps was based on a design
approved by the AEF after WW I and modified by the Corps'
Commander as described in a letter written on October 29th,
1940. The Roman numerals "IX" represent the unit's designation,
and records were not available to determine why the colors were
changed to blue and red.