History
Much of Panem's history is shrouded in mystery, though known history dates back approximately seventy-five years prior to the beginning of The Hunger Games trilogy.
Founding
An unspecified time before the current date, the face of the Earth changed and modern civilization was seemingly destroyed. It is unknown precisely what caused the "end of the world," but major landmasses changed shape as the sea level rose to unknown heights around the planet. Some time after the end of the world as we knew it, a nation was established in North America that would soon come to be known as Panem. It is unknown precisely when Panem was established and how long it has existed, though it is certain that Panem has been around for more than seventy-five years, and it's entirely possible for it to be at least a century or two old.
Panem eventually grew large enough that it was segmented into thirteen separate districts, each responsible for producing goods of a particular industry to serve the growing needs of the nation, and all operating under the auspices of Panem's oppressive Capitol. Its exact method of expansion is unknown; it is possible that some of the separate districts may have even once been smaller, separate nations that were eventually annexed by Panem.
The Dark Days
Main article: The Dark Days
Approximately seventy-four years before the events of The Hunger Games trilogy, the various districts of Panem rebelled against the Capitol, primarily due to the oppressive fashion in which the Capitol government ruled over the districts. It is unknown how long the rebellion lasted or the exact number of casualties on either side (although the Capitol claims that for every dead Capitol citizen, two rebels died), but by the end of the conflict, the Capitol had quelled the rebellion, defeating twelve of the districts and obliterating the thirteenth entirely.