What event caused the establishment of the United States Department of Homeland Security?

The United States Department of Homeland Security or DHS was established after the September 11 attacks where hijacked airplanes were sent crashing into the World Trade Center and the Pentagon by terrorists. It is a cabinet department under the U. S. Government. It is primarily responsible for protecting United States’ territories as well as protectorates from and reacts to terrorist attacks, man-made and natural disasters. It works on the civilian aspects to prepare, prevent and react to domestic emergencies specifically terror attacks.