"Homeland security is a topic that has generated a great deal of attention in the past 5 years, on both sides of the Atlantic. With the increased focus on the homeland, or domestic, security of states in the aftermath of the terrorist attacks on New York, Washington, Madrid, and London, as well as the response to disasters such as Hurricane Katrina, senior officials have been challenged to provide adequate levels of domestic security consistent with the resources that advanced Western states have available for these purposes. States have been hard-pressed to develop and equip security forces that will be able to perform the tasks required to maintain a high level of homeland security and support civil authorities in responding to catastrophes. In many instances, leaders have looked to the armed forces to carry out these key missions. Military forces bring many advantages to these challenges; they are usually well organized, trained, mobile, well equipped-and available. In many countries, there is a tradition of using military forces in support of civil authorities, a tradition that has often included a broad range of homeland security and civil support tasks. Military forces, however, are normally trained for missions that are quite different from those necessary for achieving effective homeland security. This is particularly true with regard to the use of force. While law enforcement officers are trained to use force as a last resort, soldiers are trained to use it in the first instance."
National Defense University: http://www.ndu.edu/