World War 3.
The end of mankind and life itself. The destruction of everything we know and love. The writers of fallout are full aware of the approaching doom of humanity. Fallout's premise of nuclear war devastating the world isn't far fetched. It is well within the realm of possibility, and it is going to happen. These people of power are crazy. War is a necessary means of sustaining the United States debt based economy, because we must force other countries to conduct trade with their oil using the Dollar.
Russia and china are in the process of forming their own financial system called BRICS which conducts transactions that do not involve the US dollar. This is going to be the main reason of world war 3. This is why In fallout, The United States was in a "Great War" with Russia and China.
Fallout's setting of a world completely torn apart by nuclear war is just so saddening, but yet it is enlightening because at least we see some rebuilding of humanity. Noble community's like the Brother of Steel and the NCR in fallout new vegas are good to see, even though you could argue that they have their own agenda. There is so much death and killing, people are struggling to survive day by day. All due to the lunatics in power disregarding human life.
This is what makes Fallout such a great game and sets it apart from any other franchise. It's depressing but yet amazing at the same time. It gives us a glimpse into the struggle of humanity to rebuild, to thrive.