Final answer:
The most important event following 9/11 by the federal government was the creation of the Department of Homeland Security.
Step-by-step explanation:
The most important event following 9/11 by the federal government was the creation of the Department of Homeland Security. The Department of Homeland Security was established to centralize control over various government functions in order to better control threats at home. This allowed for better coordination and collaboration among different federal law enforcement agencies to enhance national security.