Let's say that there is a really terrible disease ravishing a foreign country. All of a sudden it starts showing up in a major American city. After a day or two, thousands of people are sick, and due to the incubation period of the disease, it is likely that tens or even hundreds of thousands more have contracted it. This disease is lethal, and can be spread through simple means like handshakes or coughing. It is extremely contagious. Hell, for the sake of debate, you could even consider it something silly like "zombieism."
Should the Federal Government declare a national emergency and institute martial law in that city or state? Should the various ports and public transit options be forcibly closed? Do you think that it is possible to prevent the spread of disease, and to establish an effective quarantine, in a country of 300,000,000? Finally, do you think that armed resistance to soldiers would be a legitimate response, or would you consider such groups unintelligent terrorists?