Our understanding of homeland security continued to evolve after September 11, adapting to new realities and threats. As we waged the War on Terror both at home and abroad, our Nation endured Hurricane Katrina, the most destructive natural disaster in U.S. history. The human suffering and staggering physical destruction caused by Katrina were a reminder that threats come not only from terrorism, but also from nature. Indeed, certain non-terrorist events that reach catastrophic levels can have significant implications for homeland security. The resulting national consequences and possible cascading effects from these events might present potential or perceived vulnerabilities that could be exploited, possibly eroding citizens confidence in our Nation's government and ultimately increasing our vulnerability to attack. This Strategy therefore recognizes that effective preparation for catastrophic natural disasters and man-made disasters, while not homeland security per se, can nevertheless increase the security of the Homeland. (White House, National Strategy for Homeland Security, Homeland Security Council, October 2007, p. 3).