Before the world ended in OTL 1300, the world was stuck in a Cold War like state between the USA (OTL USA + parts of Canada and all of Mexico, minus the Oregon territory), the Franco Roman Empire (Charlemagne's Empire, ATL version of Napoleon's Empire), and the Russian Empire (OTL Russian Empire + Poland + Finland + Manchuria + Mongolia + Uyghuristan + Russian America's Alaska).
In the nuclear war, Russian Tsar Alexei II ordered the firing of several missiles into the United States of America, a world power that grew from the former United Kingdom, which was deposed by the great Charlemagne (ATL Napoleon), who conquered most of Europe on the Russian border. (This was because the USA ordered a first strike into Alaska and an invasion.) Charlemagne II and Alexei II had formed an alliance, but Charlemagne's chief minister Henrik betrayed the empire, the Holy Franco-Roman Empire, and launched nukes into Southern Scandinavia, the United States, and Siberian Russia. The enraged Alexei II ordered missiles into French Iberia and France itself, while troops marched in on the Prussian border of the French Empire. However, Charlemagne was killed on the first blast into Paris, and son Charlemagne III launched the full French arsenal out of Italy and England and into the USA and Russia. Russia also destroyed the Japanese Empire (Japanese home islands, plus what remains of China and the Indochinese area) and the Mughal Empire, as they part of the French Coalition Alliance. High Chancellor of the United States Nathanael Fawkes surrendered the United States to Russian America, but Fawkes later evacuated to Mexico and gave orders to blast out Russia, which ultimately destroyed one of the last surviving nations on Earth.