Final answer:
The document that established USNORTHCOM is not specified, but the National Security Act of 1947 laid the foundation for a unified military command. USNORTHCOM, established in 2002, would have been created through a presidential directive following the events of 9/11.
Step-by-step explanation:
The key document that established USNORTHCOM as the combatant command assigned to defend the United States is not explicitly mentioned in the provided references. However, the context suggests that the structure of the United States military commands and their roles likely stem from larger changes to the military and national security policies that have occurred over time. Specifically, the National Security Act of 1947 significantly reorganized the U.S. military and created a unified military command structure under the Department of Defense. Although this act did not create USNORTHCOM itself, it set the stage for the development of such combatant commands. USNORTHCOM was actually established much later, on October 1, 2002, by the President of the United States in response to the September 11, 2001 attacks, and its establishment would have been formalized by a presidential directive or executive order.