In the year 2066 China invaded Alaska, now in the regular Fallout Universe, the USA won this war and kept Alaska, but what if Alaska was not kept by the USA and China took over?
In the year 2070 China successfully takes control of Alaska and sets up many military camps throughout Alaska. The USA is still trying to reclaim Alaska but the Chinese numbers are too high, and they fail.
In the year 2075 China has basses up in Alaska and attacks Canada. Over the next ten years Canada surrenders and they come to an agreement with the Chinese.
In the year 2085 the Chinese are allied with the Canadians and there numbers are bigger then ever. They have a military base up in Toronto and are planning a full scale attack on Washington D.C. in the year 2100.
Throughout the next fifteen years, the Chinese are occupying all of Canada and Alaska. And throughout these fifteen years the US has tried to liberate Canada and take back Alaska but they continuously fail.
2100, The Chinese launch an unexpected attack on Washington D.C. But fortunately the US expected something like this and was already working on something that would genetically enhance human capabilities making them faster, stronger, more agile, and able to take a whole lot more bullets then a human could. These men were known as, OMEGA SQUAD.
So yeah, it is about OMEGA SQUAD, trying to save D.C.