Assuming the rise of the Japanese Empire is similar to the our world's, the empire's seeds began on March 31, 1854 with the Convention of Kanagawa. Where Commodore Matthew Perry of the United States Navy forced the opening of Japan after 220 years of national seclusion (鎖国 Sakoku). The following years would bring similar (and unequal) treaties with other Western countries and lead to political and economic turmoil. This turmoil would eventually see the collapse of the Tokugawa Shogunate and restoration of Imperial rule in 1868 under Emperor Meiji (明治維新 Meiji Ishin).
In the Meiji Era, Imperial factions resolved to modernize feudal Japan into a power that could rival those in the West (富国強兵 Fukoku kyōhei, "Enrich the state, strengthen the military"). After rapid industrialization and the modernizing of both its military and infrastructure, Japan began looking outward to expand its influence. The Japanese Home Islands were bereft of many raw materials and resources necessary to sustain modern industry, and so the Imperial government saw military expansionism as vital to the Empire's survival.
Now an industrial power, Japan moved to dominate its Asian neighbors. The Empire rendered Korea a puppet state in 1876 (later annexing it in 1910) and crushed China in the First Sino-Japanese War in 1895, acquiring the Liaodong peninsula and Taiwan. Finally, and most impressively, Japan defeated the Russian Empire in the Russo-Japanese War in 1905. securing Liaodong and Korea, as well as opening up the region of Manchuria to Japanese influence. Notably, this war marked the first time a modern, industrial European nation was defeated in a war by a non-European power.
Emperor Meiji died in 1912 and was succeeded by his son the Crown Prince as Emperor Taishō (大正天皇 Taishō-tennō). During World War I, the Empire aligned with the Allied Powers and declared war on Germany as an opportunity to grab more territory, acquiring the Marianas, the Caroline and Marshall Islands, and the Shandong province of China (German controlled). When the war was won, the Empire demanded a "Racial Equality Clause" in the Treaty of Versailles (with the implicit notion that the West view the Japanese as racially equal), this demand was outright rejected by a number of Allied Powers. This rejection played an important part in the Empire's souring of relations with the West and its reconciliation to Germany after the war.
Taisho died in 1926 and like his father before him was succeeded by the Crown Prince, who became the new Emperor, Shōwa (昭和時代 Shōwa Jidai). Conservative factions within the Imperial government grew ever stronger and more radicalized. Nationalism (always present within the Empire's history) intensified into Kokutai, which at this time, symbolized the ideal that Japan and its people were altogether superior to all other nations on Earth. The military's power grew as well, it effectively deposed civilian power in 1932 and while there was the official pretense of the Emperor in control, the true power lied with the Generals and Admirals of the Imperial Army and Navy, a situation ironically similar to that of a Shogunate.
As Hitler rose to power in Europe, the Japanese were already at war in Asia, seizing Manchuria and transforming into a puppet state, Manchukuo in 1931, then launching a full-scale invasion of China in 1937. At some point, the Empire allied with Hitler's Reich and together overcame the Allied Powers in WW2. Triumphant, the two new superpowers carved the territory of the defeated nations between them.
Flags and Emblems
The official flag of the Japanese Empire, in 1962, is still the Hinomaru, the original civil and state flag of Japan from 1870 to 1962 (a flag with a red sun-disc centered on a white field). However, by 1962, the series shows the flag, in numerous public and official manifestations, as having been largely substituted with the Kyokujitsu-ki (the Rising Sun Flag) which served, since 1889, as naval and army (with slightly variations) ensign of the Japanese Armed Forces, and it's a clearly sign and evidence of the militaristic public life of the Japanese Empire and of its policies. The flag is a white field with a red disc slightly to the hoist with 16 rays extending from the disc to the edges of the flag.
In the opening scene of season two of the series there is an Atlas briefly shown with Japanese controlled territory marked in grey. The Japanese territories include most of the areas listed above, in the scene view of south Asia and the Pacific are obscured, but also the west coast of North and South America.
Politics and PoliciesThe capital of the Japanese Empire is Tokyo, which has been the capital of Japan since 1869. Tokyo is never shown in the series but it's mentioned as a center of economic and political relations of the Greater Co-Prosperity Sphere and therefore the Japanese Block. The Japanese Empire government is described as a typical military oriented government, which still officially has His Majesty, The Emperor of Japan as Head of State, despite not having any serious political power. The monarchy does not have extensive rule, leaving the government to the military. This however, seems to have created some issues between the Japanese Military and the Emperor's Court about the general policies that the Empire must take for the future, especially in regards to the other superpower, the Greater Nazi Reich, which has a clear advantage over the Japanese Empire, in both technological and scientific development, having created the Heisenberg Device (the nuclear bomb) and even supersonic jet airliners. The main point being to either take a more aggressive position with the Greater Nazi Reich or continue a policy of "peace and balance" with the Greater Nazi Reich. On the international stage, Japan is the leading nation of the Japanese Imperial bloc, though it seems to be carrying out a more indirect method of control through membership of the Greater Co-Prosperity Sphere. Though formally independent, the Imperial Japanese Army and even the Kempeitai seem to be able to operate with impunity in member nations, and the proper Japanese consider these countries little more than colonies. The nations that are known to be members of the Greater Co-Prosperity Sphere in the Man In The High Castle universe are: Eastern Siberia, Mongolia, China, Tibet, Bhutan, Nepal, India, Burma, Indonesia, the Philippines, the Japanese Pacific States and even Australia and New Zealand. One last aspect of the Japanese Empire, on the domestic stage, is that in the Japanese Empire the Yakuza (the Japanese mafia) has grown relatively stronger than in our timeline, allowing them to be an effective anti-state force.
Japan is a monarchy, under a one-party military dictatorship.
Economics and TechnologyThe economy of the Japanese Empire is quite prosperous and with the construction of the Greater Co-Prosperity Sphere the Japanese were able to reaffirm their economic positions with the rest of the world. However the Japanese scientific development is still behind that of the Greater Nazi Reich. The Japanese Empire and all other countries of the Greater Co-Prosperity Sphere use the Japanese Yen as currency. No other information is given about the economy of the Japanese Empire.
Divergences with Our World
To be Continued...