The Constitutional Monarchy of Japan, known informally as South Japan, was the state the United States established on the Japanese islands from southern Honshu down to Kyushu, after World War II. The first emperor of the new state was Akihito, the teenage son of Hirohito, the last emperor of unified Japan, who was killed in the closing days of the U.S. invasion at the end of the war. Similarly, the Soviet Union established on the Japanese People's Republic (North Japan) on the island of Hokkaido and the northern part of Honshu, under General Fedor Tolbukhin with some Japanese Reds acting as his puppets.
In 1947, the Soviet military began building the "People's Liberation Army" in North Japan. In response, the U.S. created the "Constitutional Guard" in South Japan. Throughout 1947 and into 1948, U.S. commanders along the demilitarized zone sent reports to their superiors, expressing concern about North Japan's activities.
In June 1948, the North Japanese People's Liberation Army invaded South Japan, beginning the Japanese War. While the south's military was initially sent into retreat, the PLA's invasion was halted by U.S. forces at Utsunomiya.  From there, the U.S. and South Japanese, with bloody and hard fighting over the following year, forced the invaders back to Sendai, well north of the border.
The United States destroyed Sendai with an atomic bomb on August 6, 1949. In response, Soviet Premier Leon Trotsky ordered the bombing of the South Japanese city of Nagano three days later. The war ended in a stalemate, with a restoration of the status quo ante bellum.
With the fighting over, the South Japanese government continued to follow the U.S. government's directives. South Japan also began teaching all children English.
South Japan appears in the short story as well. The novel establishes that South Japan is a constitutional monarchy, whereas the story does not describe South Japan's government.