The United States of America is a country in North America. It is the primary setting for all of The Walking Dead, and Season One and the first part of Season Two of Fear the Walking Dead. It is bordered to the south by Mexico and to the north by Canada.
Considered to be the only world superpower left after the end of the Cold War, the United States was an economic powerhouse and a world leader in technology and entertainment, among many other areas.
From information gleaned from the show, the United States appears to have been the origin for the (eventually global) outbreak, with no reports in any other country before the initial outbreak in the U.S.
It appears that the outbreak in the U.S. began west of the continental divide and spread eastwards across the continental United States, as well as northwards into Canada and southwards into Mexico. (It should be noted that a portion of a newspaper which was released in promotional material for FTWD seems to indicate minor infections nationwide with one or two individuals involved. This would fit with what Shane said about it appearing in isolated locations).
The initial responders to the crises were local law enforcement agencies such as the California Highway Patrol and the Los Angeles Police Department, as well as field offices of the Center for Disease Control. Once it became apparent that local law enforcement was unable to handle the situation, National Guard Units were deployed (this would change to regular army units as the epidemic moved eastward). However, lack of information about the infection itself and those it affected led to these organizations gradually being overrun.
The leadership of the U.S. Military (and it is assumed, high ranking members of the Federal Government) devised Operation Cobalt in order to try and control the outbreaks. First was the establishment of "Safe Zones" within major population centers with a view to use them as springboards to reclaim infected territory. The establishment of safe zones was followed by quarantine of suspected infected individuals (and their termination if infected, with their bodies being burned). If these measures were to fail, then the final step was the large scale bombing of major urban areas to kill the infected. The bombing of Los Angeles, California was seen in the first episode of Season 2 of FtWD. The bombing of other areas was confirmed by George Geary or witnessed by other survivors (such as Shane and Lori witnessing Atlanta being bombed).
It is unknown if the Unnamed Military Camp was a part of Operation Cobalt, but it apparent role as a camp for survivors would indicate it was.By the beginning of Season 2 of Fear, it is unknown how many states have been infected, with some reports at this point claiming eleven states having declared emergencies.
According to the last broadcast from the Park Ranger at Shuteye Peak, the bombing of major cities has had major ecological consequences. By his estimation, the fires caused by the bombing will wipe out Yosemite and the Sierra Nevada within a week. (Fear the Walking Dead Radio Waves podcast).
In the Fear episode North, it can be seen that there are still individuals guarding the U.S. boarder with Mexico. Whether they are sanctioned by whatever remains of the government, ordinary citizens concerned about 'immigration' or if they are simply bandits is unknown.
By the time Rick Grimes awakes from his coma, all of mainland America (and the rest of the world) has being overrun by the walker epidemic. At this stage, it would appear that all local law enforcement agencies and the U.S. Military (apart from scattered outposts) have ceased to function. It is unknown whether or not the federal government has ceased to work. It is implied that the Continuity of Government plan was put in place, owing to a broadcast by a citizen. If elements of the federal government did survive, then they are in such poor shape that they cannot govern.
The only known member of the federal government who survived until after the apocalypse is Deanna Monroe (TV Series).