America went communist in the 1890s, much of Europe followed, and the only remaining capitalist power was the Chinese Republic under Sun Yixian. By the late 20th century, excessive industrialisation and poor environmental safeguards were causing major problems in the communist nations (i.e. everything except China), so they blamed China, declared war on it, and nuclear war resulted. The communist nations fell to infighting, but the global ecological collapse did more damage. It is now 2021, and most survivors outside China are TL3 at best. The Chinese survivors are around TL4, trying to hang on to what little TL7 tech they have left.
Before the war, China had a few TL10 experiments, and, just before the war started, had sent off an Orion drive starship to an unknown destination. Naturally the survivors don't know what happened to it.